Friday, March 21, 2014

Gravitational Waves Seen in the Polarization of Light From the Big Bang

THE COSMIC MICROWAVE BACKGROUND

The oldest light we can see in the Universe is called the cosmic microwave background (CMB) and it is the relic light from the Big Bang.  While this light is old, it isn't quite as old as our Universe.  Before an event called recombination, the Universe was not transparent to light, so the light couldn't propagate very far before being absorbed.  Recombination happened about 380,000 years after the Big Bang and the light from this time is what we observe in the CMB.

Everywhere we look on the sky, the frequency of this microwave light is very nearly the same.  Since heat can be transmitted through radiation (such as microwaves), we can characterize this light to have a temperature of about 3 K (or about 3 oC or 5.4 oF above absolute zero - the coldest anything in the Universe can be).  Why this temperature is the same everywhere on the sky doesn't immediately make sense since the heat hasn't had enough time to be transferred across the Universe.

The slight variations in the CMB temperature from opposite sides of the sky as measured by 9 years of data from the WMAP mission.  The fluctuation in the CMB temperature is measured to be ± 0.0002 oC (0.00036 oF).  [Source: Wikipedia]

The CMB is almost 14 billion light years away from us.  This is approximately the age of the Universe.  But there is no way for light to transfer heat from one side of the Universe (14 billion light years from us) and reach the other side (14 billion light years away from us in the opposite direction) since this would take about 28 billion years of travel time or twice the age of the Universe!  So there is no way that these widely separated parts of the Universe should have the same temperature if the Universe has expanded in a continuous way since the Big Bang.

To explain why the CMB is essentially in thermal equilibrium in every part of the Universe, something extraordinary needed to happen...


INFLATION

Almost immediately after the Big Bang, it is believed that the Universe entered a period of extremely rapid expansion called inflation.  This began at about 10-35 seconds after the Big Bang and the Universe proceeded to expand its volume by about 80 orders of magnitude (that's a 1 followed by 80 zeroes) in a fraction of a second.  During this time, gravitational waves would have originally been produced on a quantum mechanical scale and then blown up to cosmological scales during inflation.  The gravitational waves from the Big Bang are exactly these fluctuations in space-time that are still vibrating from the period of inflation.  (The wavelength now is its original wavelength, i.e. about 1% of the size of the Universe as it was then, stretched by the amount the Universe has expanded since then.)


EVIDENCE OF GRAVITATIONAL WAVES IN THE CMB

Since gravitational waves were able to propagate through the early Universe long before light was, it is expected that there is evidence of these gravitational waves contained within the CMB.  We expect to see this in a special kind of polarization of the CMB (where polarization refers to the rotational orientation of the light waves).  There should be 2 kinds of polarization in the CMB, E-mode and B-mode.

A graphical history of the Universe showing when gravitational waves would have been created and how they affect matter along with density waves and their affect.  The effects that gravitational waves have on mattert cause B-mode polarization in the CMB while density waves are the primary contributors of E-mode polarization.  [Source: Wikipedia]

E-mode polarization means that the orientation of the polarization should not change as you move in a straight line.  B-mode polarization means that the rotation of the polarization changes or "curls" around itself.  The E and B in these mode names refer to how electric (E)  and magnetic (B) fields behave: a single charge will have an electric field pointing radially away from a single change while a magnet always have 2 poles causing the magnetic field to always curl back to the magnet.  The E-mode polarization in the CMB provides information about the fluctuation of density in the early Universe.  Because gravitational waves alternate, compressing space in one direction and expanding it in the orthogonal (at a right angle) direction, they caused the "curling" B-mode polarization.

Graphical illustration of the polarization patterns for E-modes and B-modes.  Note that B-mode patterns can be characterized by "rotating" clockwise or counter-clockwise while the E-modes cannot.  [Source: Press conference screen grab]

An experiment called BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) announced to the world a few days before this post (17 March 2014) that they did indeed detect the B-mode polarization in the CMB.  The results are cataloged here.


The above image is the polarization of different points in the sky they observed from the South Pole.  The red colored areas are where the B-modes can be classified as clockwise and the blue colored areas are where they can be classified as counter-clockwise.

THIS IS THE IMPRINT OF GRAVITATIONAL WAVES FROM THE PERIOD OF INFLATION!

Any time scientists think they found something that they wanted to find, we immediately set to trying to disprove what we found.  (This is discussed on this blog in regard to LIGO with the blind injections known as "The Big Dog".)  After thorough vetting and analysis of this work, it has been determined that the chance of this B-mode signal has a chance of 1 in 3.5 million of being a false detection. 


WHAT THIS MEANS FOR LIGO AND SIMILAR DETECTORS

This discovery of the imprint of gravitational waves on the CMB further hints at the promise that gravitational-wave astronomy with detectors like LIGO will have in the future.  Their discovery in no way diminishes the potential of LIGO and gravitational-wave astronomy - instead it increases its promise.

LIGO seeks to work like a gravitational-wave radio and record the gravitational-wave signals directly.  (This analogy is discussed in more depth on this blog here.)  For this analogy, the information about what made the gravitational wave is the music being carried on the radio wave (the gravitational wave in this analogy).  In this sense, LIGO will be making a distinctly different kind of detection than BICEP2 did.  We will be directly recording a gravitational wave as it passes by Earth and BICEP2 detected the imprint of gravitational waves on the CMB.

Also, LIGO looks for a wider range of gravitational waves.  While we also look for the relic gravitational waves from the Big Bang which we call stochastic gravitational waves, we search for three other kinds: continuous, inspiral, and burst.  (These are described in more detail on this blog here.)  This broad range of gravitational waves that detectors like LIGO will be able to "see" will allow gravitational waves to tell their own story of how they were made; perhaps from the collapse of a star into a black hole or the merging of two stars into one, or the echoes of the birth of the Universe.  We will not be seeing the evidence of gravitational waves that is imprinted onto light, but collecting information from the gravitational waves themselves.

As a side note:  Kip Thorne, a physicist who has pioneered work in general relativity and gravitational waves, made a prediction in 2006 of what detections will be made with gravitational waves in the next 50 years:
"Over the next 50 years, gravitational waves from the big bang will be detected, first indirectly by the imprint they leave on the cosmic microwave radiation and then directly, by space-based gravitational wave observatories."
 You can read the rest of his prediction on NewScientist.com.

Read LIGO's official congratulatory statement on the BICEP2 results to the ligo.org web page.

LSC Congratulates BICEP2 Colleagues


18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest.
This result highlights the unique role that gravitational wave observations will play in understanding the universe in the coming years, demonstrating the possibility to study the earliest time in the evolution of the Universe, and the physics of the correspondingly high energies, using gravitational waves. Direct measurements of the cosmological gravitational waves at a variety of frequencies will be necessary to fully understand the physics of inflation. Furthermore, when Advanced LIGO gravitational-wave detector comes online in the second half of this decade, we anticipate it will directly measure gravitational waves created by the most violent compact astrophysical sources in the universe --- colliding neutron stars and black holes as well as supernovae --- opening an entirely new window onto the universe through gravitational-wave astronomy.
- See more at: http://www.ligo.org/news/bicep-result.php#sthash.MeRey6C9.dpuf
18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest.
This result highlights the unique role that gravitational wave observations will play in understanding the universe in the coming years, demonstrating the possibility to study the earliest time in the evolution of the Universe, and the physics of the correspondingly high energies, using gravitational waves. Direct measurements of the cosmological gravitational waves at a variety of frequencies will be necessary to fully understand the physics of inflation. Furthermore, when Advanced LIGO gravitational-wave detector comes online in the second half of this decade, we anticipate it will directly measure gravitational waves created by the most violent compact astrophysical sources in the universe --- colliding neutron stars and black holes as well as supernovae --- opening an entirely new window onto the universe through gravitational-wave astronomy.
- See more at: http://www.ligo.org/news/bicep-result.php#sthash.j7HjXzk2.dpuf
18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest.
This result highlights the unique role that gravitational wave observations will play in understanding the universe in the coming years, demonstrating the possibility to study the earliest time in the evolution of the Universe, and the physics of the correspondingly high energies, using gravitational waves. Direct measurements of the cosmological gravitational waves at a variety of frequencies will be necessary to fully understand the physics of inflation. Furthermore, when Advanced LIGO gravitational-wave detector comes online in the second half of this decade, we anticipate it will directly measure gravitational waves created by the most violent compact astrophysical sources in the universe --- colliding neutron stars and black holes as well as supernovae --- opening an entirely new window onto the universe through gravitational-wave astronomy.
- See more at: http://www.ligo.org/news/bicep-result.php#sthash.MeRey6C9.dpuf
18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest.
This result highlights the unique role that gravitational wave observations will play in understanding the universe in the coming years, demonstrating the possibility to study the earliest time in the evolution of the Universe, and the physics of the correspondingly high energies, using gravitational waves. Direct measurements of the cosmological gravitational waves at a variety of frequencies will be necessary to fully understand the physics of inflation. Furthermore, when Advanced LIGO gravitational-wave detector comes online in the second half of this decade, we anticipate it will directly measure gravitational waves created by the most violent compact astrophysical sources in the universe --- colliding neutron stars and black holes as well as supernovae --- opening an entirely new window onto the universe through gravitational-wave astronomy.
- See more at: http://www.ligo.org/news/bicep-result.php#sthash.MeRey6C9.dpuf

LSC Congratulates BICEP2 Colleagues


18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest.
This result highlights the unique role that gravitational wave observations will play in understanding the universe in the coming years, demonstrating the possibility to study the earliest time in the evolution of the Universe, and the physics of the correspondingly high energies, using gravitational waves. Direct measurements of the cosmological gravitational waves at a variety of frequencies will be necessary to fully understand the physics of inflation. Furthermore, when Advanced LIGO gravitational-wave detector comes online in the second half of this decade, we anticipate it will directly measure gravitational waves created by the most violent compact astrophysical sources in the universe --- colliding neutron stars and black holes as well as supernovae --- opening an entirely new window onto the universe through gravitational-wave astronomy.
- See more at: http://www.ligo.org/news/bicep-result.php#sthash.MeRey6C9.dpuf
18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest. - See more at: http://www.ligo.org/news/bicep-result.php#sthash.j7HjXzk2.dpuf
18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest. - See more at: http://www.ligo.org/news/bicep-result.php#sthash.j7HjXzk2.dpuf
18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest. - See more at: http://www.ligo.org/news/bicep-result.php#sthash.j7HjXzk2.dpuf
18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest. - See more at: http://www.ligo.org/news/bicep-result.php#sthash.j7HjXzk2.dpuf
18 March 2014 - The BICEP2 Collaboration result, if confirmed, is a landmark discovery in cosmology, allowing us for the first time to peer back almost to the moment of the Big Bang through the observation of the imprint of primordial gravitational waves on the cosmic microwave background. The LIGO Scientific Collaboration congratulates our BICEP colleagues on their accomplishment and will further follow discoveries and implications of these observations with great interest. - See more at: http://www.ligo.org/news/bicep-result.php#sthash.j7HjXzk2.dpuf

WHAT THIS MEANS FOR COSMOLOGY

The BICEP2 results do much more than suggest or support that inflation happened: it gives us some information about what happened during inflation.  The strength of the signals observed here informs us on the energy involved in inflation.  The ratio of the strength of the E-modes to the B-modes (a value referred to as r and measured here to be r = 0.2) is proportional to the energy density of the Universe at the time of inflation and this is consistent with energies needed in some of the grand unified theories (GUTs) (this is where the strong, weak, and electromagnetic forces become indistinguishable).

The BICEP2 results also serve to constrain the theories of what happened during inflation.  Several of these have been ruled out (e.g. large field inflation models are now highly unlikely).

Ultimately, these results need to be reproduced and refined by coming experiments.  This doesn't mean that the scientific community isn't confident in BICEP2's results, but science needs to be reproducible.  And in reproducing results, they are often refined and expanded upon.

This truly is an exciting time to be a scientist!


See Also:

Wednesday, December 18, 2013

Silver and Gold: Clues to the History of Our Solar System

It's that time of year when many of us are buying gifts for our loved ones.  And there's a song from the old Rudolph the Red-Nosed Reindeer TV special that comes to mind (sung by the character Yukon Cornelius):  "Silver and gold, silver and gold.  How many wonders can one cavern hold?..."    But have you ever stopped to think about where your jewelery came from?  I don't mean how your silver and gold was mined, but how was it created?  It's much more interesting than anything you've probably imagined!

SILVER & GOLD

Gold crystal (image from Wikipedia)

First, let's go back to the birth of the Universe:  BANG!  (That was the Big Bang.)  Sometime between 10 seconds and 20 minutes from now, the first atoms will be formed (this is called Big Bang nucleosynthesis).  These first atoms were of the simplest elements in the periodic table; mostly hydrogen and some helium and lithium.  Even today, the most common elements in the Universe are hydrogen and helium.  A few hundred million years later, clouds of this material will collapse to form the first stars.  This birth is marked by the ignition of nuclear fusion: the smashing of lighter elements together to make heavier ones.  All stars start by smashing 2 hydrogen (H) atoms together to make a helium (He) atom.  Eventually, the star will start to run out of hydrogen and will start to fuse helium to make beryllium (Be) and carbon (C).  The largest stars will continue fusing atoms together until the product element is iron (Fe).  For elements lighter than iron, smashing two lighter elements together to make a heavier element will release some energy.  For elements heavier than iron, breaking an atom into two lighter atoms will release energy: this is called nuclear fission.  So, the heaviest element that any star can normally make during its life is iron.  Heavier elements like silver (Ag), gold (Au), and platinum (Pt) and many other elements can only be created in the most extreme environments such as a supernova explosion that marks the death of a star.  Therefore, the metal in our jewelery is truly a relic of a star that died before our Sun was born.

Our Sun and solar system was born from the supernova explosion of an older star that died and, in doing so, seeded the material that makes up our solar system with heavier elements (this is the nebular hypothesis).  For example, since the Earth is not a star it cannot perform nuclear fusion.  Yet the core of our planet is primarily iron and nickel and had to be made in a previous star.  This is what Carl Sagan meant in his famous quote, "We are all starstuff."


WHAT ABOUT DIAMONDS?

Diamond set into a gold ring (image from Wikipedia)

 While I'm on the topic of the exotic origins of jewelery (and many other things), let me talk a little about why our Sun may turn into a giant diamond.

First off, a diamond is made from carbon which our Sun is capable of making much later in its life (more on this in a little bit).  Carbon is everywhere around us - pretty much every molecule that isn't water in your body has at least one carbon atom in it (this is the definition of an organic compound).  What makes a diamond special from all of the other common forms of carbon is that it has a very specific crystal structure that is only formed under extreme pressure and heat.  (Any mineral that has the same chemical structure but a different crystalline organization is called a polymorph.)  So, the diamonds that are formed naturally on Earth started as a carbon deposit that transformed into a diamond under the extreme pressure and temperatures found beneath the surface of the Earth.

What does this have to do with our Sun?  Well, right now the Sun is busy turning hydrogen into helium.  It will eventually (in about 5 billion years) start making heavier elements, but only much more massive stars ever have enough oomph (enough mass to create the most extreme pressures) to fuse elements all the way up to iron.  Our Sun is not one of those stars (which is good since those massive stars live comparatively short lives).  The Sun will only be able to produce up to carbon and oxygen (O) before fusion stops and the outer layers of the Sun are blown into the interstellar medium to seed future stars and solar systems.  What is left behind is called a white dwarf composed of carbon that has been exposed to intense pressure and temperatures.  That means that the white dwarf our Sun leaves behind may very well be a diamond!

A white dwarf that could be a massive diamond has already been observed and nicknamed "Lucy" (after the Beatles song "Lucy in the Sky With Diamonds").  It is more massive than the white dwarf our Sun will leave behind, but seems to be created by the same processes we expect for our Sun.  All of this is very promising for a diamond to be the memorial for our Sun (and solar system).


WHAT DOES THIS HAVE TO DO WITH GRAVITATIONAL WAVES?

Directly, not much.  However, this past August it was confirmed that a short gamma-ray burst was the result of a kilonova, the collision of two neutron stars that releases a massive amount of energy (read more about this here and see my comments towards the end of the article).  Neutron stars are the remnants of dead stars that were much more massive than our Sun (about 10 to 40 times the mass of our Sun) that died in a supernova.  Those past supernovae would have created silver, gold, and everything heavier than iron and seeded the interstellar medium for future stars and solar systems.

These kilonova sources should produce copious amounts of gravitational waves, however, there has never been a short gamma-ray burst (like the one I mentioned above) detected close enough for LIGO to see it.  Once Advanced LIGO is complete (soon!) the closest short gamma-ray burst will be just on the edge of the distance we expect to be able to detect these kinds of gravitational waves.  But the great thing about what we do is that we really don't know how strong the gravitational waves from the actual explosion will be so they could very well be detectable in the future.

Fingers crossed!

Tuesday, October 8, 2013

Living LIGO's Belated 3rd Anniversary

This past Saturday, 5 October, was the 3rd anniversary of the Living LIGO blog (you can see the first ever post here).  I remember that because it also happens to be my birthday - I started this out of a desire to do something I always wanted to so what a better day to start than your own birthday.  (If you must know, I've just celebrated the 6th anniversary of turning 29.)

I know it has been a long time since I've posted.  I've been teaching at LSU and doing research at LIGO.  On paper my life looks great but the reality is that there are many details, both personal and professional, that have added up to me not being in a great place for a while.  I've been getting my jobs done but after that I've been pretty exhausted, at least mentally.  This has happened to me before, so I thought I would direct you to my thoughts on what it's like being down but getting up again anyway here.  (Also see my last section below: "A WORD OF ADVICE...".)

But there is one thought that has come up many times in the last few months:  "I'd like to write a blog post on that."  There are many different things, like continuing the series of posts I've started about methods of looking for gravitational waves or telling the story of where silver and gold come from (as in, how did it come to be on Earth).  So, I am going to dig myself out of my slump and get back on my metaphorical horse - starting now!


WHAT I DID THIS SUMMER

Let me tell you a little about what I've been doing since I've last posted.  I got to go to a large meeting called the GR20/Amaldi10 Meeting in Warsaw, Poland (where almost 850 gravity theorists and experimentalists gathered for this joint meeting) and gave 2 invited talks.  The first was a formal talk on outreach skills and media (featuring this blog) and the other was less formal and was on the benefits searching for gravitational waves can bring mankind focusing on spin-off technology (I've written about this before here).

This is a picture of the gates of the Uniwersytet Warszawski where GR20/Amaldi10 took place.

A view through the gates at my colleagues on a coffee break in the distance.

One of the best parts of meetings like this is that the meeting dinner is usually somewhere a normal person couldn't go.  Our dinner was at the in the Royal Castle in Warsaw.  And this is my husband and I on the lawn beforehand:

My husband, Derek, and I on the lawn behind the Royal Castle in Warsaw, Poland.

Just before my trip to Warsaw, I took a short holiday to Paris, France.  There was a debacle with lost luggage and then wrong luggage being delivered to us, but outside of wearing the same clothes for a few days (this is why I always pack extra underwear in my carry-on luggage), my husband and I had a great time just relaxing and wondering around.  My new phone takes panoramic pictures and this is a good one I got of the Louvre:

Panoramic view of the Louvre (Paris, France)
Click on the picture for a larger view.

Once I returned home to the United States, I had tons of work to do.  I've talked before about how LIGO has been testing its data infrastructure to prepare for Advanced LIGO here.  We are then had our 4th software engineering run; that means that I had to improve upon the gravitational wave simulation software I wrote to perform better, faster, and incorporate more features.  It was a bit stressful come the deadline, but it all got done and turned out well.


... AND THEN SCHOOL STARTED AGAIN

Then school started again.  I am teaching (at LSU) the first semester of physical science (sometimes referred to as "Physics for Poets" since it is more conceptually based than mathematically focused).  Most of my students in this class (this year my lecture has only about 100 students) are elementary education majors.  Some might think that teaching an "easy" class like this would be, well... easy.  But it is far from it.  The less math you can rely on to teach the subject matter, the better you have to be as a teacher in communicating what the math means.  Since I love challenges like this, this is one of my favorite classes to teach.  I am also team teaching a junior/senior level class on Science Methods for secondary education pre-service teachers majoring in science or math.  This class shows them how science is done by doing experiments using the scientific method, analyzing their data and reporting it in both papers and presentation (since these are the two main means that scientists communicate with each other).

At LIGO, I am working on a paper with a group of other LIGO scientists who are looking for gravitational waves from supernovae that may have occurred while LIGO and/or other gravitational wave observatories were in operation (before the advanced detector upgrade began).  And, as always, I continue to refine my gravitational wave simulation software.


A QUICK WORD OF ADVICE...

I'll be writing again soon (probably next week).  When I first started this blog, I promised you a peek into the life of a working scientist.  Lately I've been answering lots of questions about gravity and how to look for gravitational waves.  But since there has been something major going on in my life, and it kept me from writing my blog as I would have liked, I wanted to share that with you.

I am lucky since even though I know I have times when depression can get the better of me, I have wonderful support from my husband, friends, and family.  I'm not sure why, but they all seem to love me even when I can't stand to be around myself. 

For anyone reading this who has issues with depression and/or anxiety:  Don't fool yourself that everything you are feeling inside is not affecting you because you may be able to keep it together and have others think you are happy.  This will eat at you and everything you are feeling will come out sometime (and usually at the least opportune time).  If you are sad or anxious for long periods of time, even if it's on-and-off, find some help.  It is not weak to seek help (I've been told that before and it's usually by people who need help for themselves and are too afraid to get it).  It takes an inner strength to admit when you are hurting and need a hand up, an ear to listen to you, or a shoulder to cry on.  If that's not enough, talk to your doctor.  Not once has a doctor been anything but 100% supportive of me when I've gone to them seeking medical help.  NOT ONCE did they look down on me, or suggest that my feelings will pass, or that I need to "buck up".  With support and help, I've always clawed my way back to feeling like a normal person.  You can too!

Sunday, June 30, 2013

Methods of Detecting Gravitational Waves I: Resonant-Mass Detectors

As the name of this blog suggests, I use LIGO and similar gravitational-wave detectors (like Virgo and GEO).  These detectors are all interferometric detectors meaning that they use the interference of light to measure gravitational waves.  But interferometers were not the first means used to look for gravitational waves...

WEBER BARS

In 1966, Joseph Weber of the University of Maryland constructed a gravitational-wave detector that consisted of a very precisely machined cylinder of aluminum 2 meters long and 1 meter in diameter.  The idea was that when a gravitational wave passed over the bar at a specific frequency, the bar would start to ring like a bell.  This "ringing" frequency, also called the resonant frequency, for Weber's bars was 1660 Hz (cycles per second).

Weber working on one of his bars at the University of Maryland, c. 1965.

The way these bars are be used to detect gravitational waves involves the phenomenon of sympathetic resonance.  This is when the vibration of something external to an object matches its resonant or "ringing" frequency and causes it to begin vibrating.  Even after the external vibration stops, the now vibrating object will continue to ring like a bell (and eventually stop ringing just like a bell as well).




Now, when a gravitational wave at or very near to 1660 Hz passes by one of Weber's bars, it will stretch space in one direction and compress it in the perpendicular direction, much like illustrated in the animation below:


This stretching and compressing is the vibration that makes the bar 'ring'.

One property of gravitational waves is that even the strongest of them that we can hope to detect on Earth are exceedingly weak.  Because of that, any ringing of the bar will be too small to hear or even to detect using normal ways of measuring vibration.  Instead, crystals that produce an electric voltage when stretched or compressed (called a piezoelectric crystal) were mounted around the bar.  Measuring the voltage from these crystals is also measuring the motions of the bar that could be from it ringing.

But just like there are many things other than gravitational waves that can cause noise in the data that LIGO collects, there were many other things that could cause the motion of Weber's bars.  Even the vibration of the aluminum atoms in the bar due to their temperature (Weber's bars were kept in a vacuum at room temperature) created significant noise and limited how small of a gravitational wave they could detect.  Ultimately, they were limited to a strain (which is defined to be the change in length divided by the original length of an object) of about 10-16.  To give this scale some reference, a "large" gravitational wave to LIGO would produce a strain of about 10-21 and we think we can expect a gravitational wave this large about once every 10 years!  But, it is still possible that Weber's bars could have detected gravitational waves...

By 1969 Weber thought that he may have detected gravitational waves with his bar detectors and continued to make several claims over the years but none were regarded as significant enough to declare that the first elusive gravitational wave had truly been directly observed.  These claims were ultimately not accepted for many reasons including that other groups were not able to reproduce his rate of detections which Weber was claiming to be up to several a day.  Weber lost the financial support of the National Science Foundation (which now funds LIGO) after a disputed claim of the detection of gravitational waves from a supernova observed in February 1987 (SN1987A).

One of Weber's original bars on display at the LIGO Hanford Observatory in August 2004.

There are many more interesting details surrounding Weber's career that I will write a post on later.  But while the controversy surrounding his claims of detection do not seem to cast him in a favorable light, it was through his work that others became inspired to look for gravitational waves in different ways, including using an interferometer like LIGO does.  Joseph Weber is truly the father of the search for gravitational waves!


MODERN BARS

Since Weber, there have been many advancements in using resonant-mass bars to detect gravitational waves.  Most bars today are made from new aluminum alloys, are cryogenically cooled to reduce the noise from the bar's thermal vibrations, have mechanical means to amplify the vibration, and piezoelectric crystals have been replaced with even more sensitive motion sensors.  Different shapes (like spheres) have also been used to increase sensitivity to gravitational waves coming from different directions because bars are most sensitive to gravitational wave directly above or below the bar.


COMPARISON TO DETECTORS LIKE LIGO

Resonant-mass gravitational-wave detectors like Weber's bars and interferometric detectors like LIGO look for the same effect: the stretching and compressing of space caused by the gravitational wave.  Bars are far less expensive than detectors like LIGO but are only sensitive to narrow ranges of gravitational wave frequencies.  Bars are also only sensitive to small portions of the sky at once where detectors like LIGO are sensitive to most of the sky at once (including the sky on the other side of the planet).  But because of how each of these detectors look for gravitational waves, resonant-mass detectors are likely to only be sensitive to the strongest gravitational waves.


THE DETECTORS RIGHT NOW

If you are interested in the operational state of gravitational-wave detectors (including resonant-mass detectors) click hereAURIGA and NAUTILUS are both bar detectors while GEO 600, LIGO, and Virgo are interferometers.

Tuesday, May 21, 2013

Here's Your Chance to Help LISA Happen!


A depiction of a LISA satellite with its lasers.

I've written about LISA several times on this blog and most of those times the news hasn't been good.  Today I have a bit of encouraging news and a way for you to help!

For readers who've recently found Living LIGO, LISA is a space-based gravitational wave detector that will be sensitive to lower frequency gravitational waves than any Earth-based counterpart ever will be.  Gathering gravitational-wave information from low frequencies will help complete the picture that gravitational-wave astronomy can paint; Earth-based detectors are really only sensitive to the "death throes" of violent astrophysical interactions while LISA will be sensitive to these same sources in their youth.  This youth stage is so long that the predominant noise source for LISA is continually measuring the gravitational waves from these young sources coming from all over the sky at the same time.  This is called the "confusion limit" and it is like trying to listen to a conversation on the other side of the room at a busy party.  This can be overcome, but what a wonderful problem to have! The noise you measure is really just measuring so many gravitational waves at the same time that they mix together!

You can follow the unfolding of the LISA drama through these Living LIGO posts:


In the "News on LISA" (25 May 2012) post, the statement from the eLISA Consortium was that they were going to push for the next launch opportunity.  The European Space Agency has initiated the process to choose candidate missions for the next launch.  The eLISA mission team is looking for your support! 


HOW YOU CAN HELP...

Please go to the eLISA - Make History page and sign your name as a supporter.  Anyone from around the would can lend their support.  If you are a scientist, you can also opt to have your name and institution listed in the eLISA white paper.  (You can see the ESA's call for white papers here.)

If you support gravitational-wave science at any level, please consider putting your name on the list!  Let's show the world that there is true support for this science!

HURRY!  There are only 2 days (or less) to add your name to the list of supporters! 
eLISA - Make History



NEW LISA YOUTUBE CHANNEL

The LISA Mission has also created their own YouTube channel and is starting to post some truly exceptional animated educational videos.  Right now, two are available:


Gravity Ink. - Einstein's Gravity (Episode 1)



Gravity Ink. - The Future of Astronomy (Episode 2)


Consider subscribing to their YouTube channel to keep up on new installments (I will most likely feature them here too!). 

Friday, March 29, 2013

Black Holes 101

Most of what I discuss on this blog has to do directly with gravitational waves.  This time I'd like to talk about one of their most talked about exotic sources: black holes.  Black holes are an exemplary source because they are highly concentrated mass.  Just add a touch of accelerated motion and gravitational waves are emitted in abundance (well, it's not quite that simple and "abundance" is a relative term, but you get the idea).  But what are the fundamental concepts that add up to the existence of black holes?  That's what we are focusing on now.


1.  Escape velocity

You've probably noticed that the harder you throw an ball straight up in the air, the higher it goes.  We also know that the farther away the ball gets from the Earth, the lower the gravitational attraction is between the ball and the Earth.  When you connect these two concepts, you can imagine that there is a speed at which you can throw the ball up and it will never come back down.  This is called the escape velocity:


We have discussed escape velocity before on this blog (specifically when discussing the conditions an object must have to be 'eaten' by a black hole).  In this equation, G is the gravitational constant, M is the mass of the 'thing' you are trying to escape, and r is the distance you are from the center of the 'thing'.  The bigger the mass of the 'thing', M, is, the faster the object must be thrown to escape it, ve.

Going back to throwing a ball up in the air from the Earth's surface, you would need to throw that ball about 25,000 mph so that the ball would not come back to the Earth (good luck with that)!


2.  The speed of light is the universal 'speed limit'

Nothing can travel faster than the speed of light (in a vacuum), represented by c.  No matter, energy, or information about the Universe can travel faster than that.  That is pretty much the long and the short of this concept.

[Source: Knight Science Journalism at MIT blog]

This 'speed limit' comes from Einstein's special relativity and the effect of simultaneity.  Perhaps I will write a longer post about this in the future, but all that is important now is to recognize that if something needed to travel faster than the speed of light to communicate information to you, then you are never going to know about it.


3.  Light is affected by gravity

Light is both a particle and a wave.  As a particle, it has no mass.  Since gravity acts between two masses, it may be surprising that light can be affected by gravity at all!  But it does.  This effect is called gravitational lensing and I've written about this previously here


CONCLUSION:  How these concepts form a black hole

The simplest black hole is called a Schwarzschild black hole.  This is a black hole that has no electrical charge and is not rotating - it's just "there" meaning that there won't be anything to complicate our black hole situation.  For now, let us think of our black hole as having mass but no volume.  You can think of this as being the ultimate implosion.  Since this mass has no volume, there isn't any surface to it.  Eventually, as we get closer and closer to where the mass is centered, the escape velocity will become so large that the escape velocity will be greater than the speed of light.  And since light is indeed affected by gravity, that means that nothing will be able to escape the black hole.  We can even figure out what this distance is from the equation for escape velocity by setting ve to the speed of light, c, and solving for r (the distance away from the mass where the escape velocity equals the speed of light):


This distance from a black hole where light will not be able to emerge from a black hole is also called the event horizon and this sphere around the black hole is what is being referred to when we talk about the size of the back hole.  For this simple Schwarzschild black hole, it is also known as the Schwarzschild radius.  You can also think of this radius as how small a mass would have to be to become a black hole.

So, how big would the Schwarzschild radius be for some things we are familiar with?  Well, for a black hole with the mass of our Sun it would be just about 3 km or about 1.9 miles.  For a black hole with the mass of the Earth it would be 8.87 mm or a little under 11/32" (0.349 in).

Thursday, February 28, 2013

Lessons From My Childhood on How to Teach

One of the best parts of my job is getting to do outreach.  This is going out and teaching the public about the research that I do.  Since I love what I do and those that I encounter are usually interested in what I have to say or they wouldn't be there (like you wouldn't be reading this if you didn't want to), it is almost always a rewarding experience all around.  However, I had some childhood experiences with outreach that were, well, a little traumatic.  However, they have taught me lessons that I use every time I teach whether in the classroom, engaging the public at LIGO, or writing for you.





"HAIR-RAISING" TRAUMA

Ever since I was a young child, I've always known that where I am now is where I wanted to be.  That is, I've always known that I wanted to be a physicist or an astronomer.  Of course, that's not what I said; I wanted to be an astronaut since that is the hero job for the physical sciences.  My family has also been supportive of me and one of my favorite things to do was go to the planetarium.  At the time, I lived outside of Pittsburgh, PA and we would go to the Buhl Planetarium (before it became part of the newer Carnegie Science Center - the building is now part of the Children's Museum of Pittsburgh).

Front entrance of the Buhl Planetarium in Pittsburgh, PA. [Source: Wikipedia]

On the fateful trip in question, I was no more than 7 or 8 years old and I was watching a demonstration in between planetarium shows with my father.  The presenter asked for a volunteer from the crowd, preferably with long fine hair.  The next thing I felt is my father's hand on my back pushing me forward.  I wasn't interested in being the center of attention, but the presenter thought that I would be perfect for the role.

She called me forward and had me stand on a plastic milk crate beside a metal dome that was bigger than my head.  She told me that I was going to have to hold on the the metal dome with one hand but I was not to do a list of things or I would get hurt.  Then I was worried.  She had me put one hand on the dome and turned the machine on.  It made a lot noise and I feel an odd tingling over my skin.  Then I was scared.  The presenter was very happy about everything and told me to shake my head.  I did so timidly.  Then she encouraged me to shake my head with more vigor.  I shook the heck out of my head so she would leave me alone and I could be done with all of this.  Then EVERYONE who is watching this demonstration WAS LAUGHING AT ME.  Then they applauded as the machine was turned off and I was helped down from my perch and left to think I was being laughed at.

The machine with the big metal dome attached to the top.  I later discovered that this is a Van de Graaff generator.  [Source: UMN Physics department]

It wasn't until I was in middle school that I figured out why everyone was laughing at me.  That machine was a Van de Graaff generator and it deposited static electricity on me.  The warnings that worried me were to prevent me from getting "zapped" and everyone was laughing at me because my hair was standing on end.  The harder I shook my head, the more the static electricity made my hair stand out.  A lot like this:


WHAT WENT WRONG?:  The presenter didn't show me what I looked like in a mirror (as is featured in the clip above) or tell me what I looked like.  I had no idea why everyone was laughing at me or what the point of the "hair raising" demonstration was.  Without this knowledge, I walked away from the experience thinking that everyone was really laughing at ME and not the effects of static electricity.

LESSON LEARNED:  If you use a volunteer in a demonstration, make sure that they understand what is happening.

I don't have many occasions where I need a volunteer for a demonstration, but when I do I make the volunteer the focus of the demonstration so that, at the very least, they walk away understanding what happened. 

Read more about how Van de Graaff generators work.





SWINGING FOR A "BREAKTHROUGH"

When I was too young for school, I wanted to be a big girl and play school.  Even then I loved science.  One day I convinced my father to play school with me.  Using the sliding green chalk board doors on my toy box, my father taught me about the layers of the Earth.

A toy box much like the one my father used to play school with me.  [Source: It's Still Life blog]

The Earth's layers can be generalized into 4 main layers: the crust at the surface where we live, then the mantle, and finally the outer and inner cores.

Diagram showing the layers of the Earth.  [Source: About Earth blog]

I was told that the crust was very thin and the mantle is hot molten rock (magma) [note: only the mantle near the outer core is molten but the mantle under the crust is about 1000oF so I equated that to "molten" too as a child].  I'd seen documentaries about volcanoes on television and knew what "molten rock" meant.  This completely changed the way I saw the swing set in my back yard.  Why?  Well, have you noticed the divot under the swings where you drag your feet to slow the swing to a stop?  I saw that as eating away at the crust and I was afraid that I would break through to the mantle and sink my feet into molten rock!  I know that it really isn't logical since I'd seen deeper holes before and there was nothing but dirt at the bottom, but I was a little kid and didn't think like that.  Anyway, I then was afraid of breaking through the crust if I dragged my feet and I was too chicken to jump off.  That left me sitting on the swing waiting for it to slow down on its own.  The wait took a lot of the fun out of swinging!

WHAT WENT WRONG?:  The scale of "thick" and "thin" was not established.  When I heard that the crust was thin, I defined for myself what "thin" was.  I assumed it was only as deep as I could dig through it.  What "thin" really meant is compared to the size (radius) of the Earth.

For the record, the radius of the Earth is almost 4,000 miles and the crust is up to about 22 miles.  Since 22 miles is much, much less than 4,000 miles, the crust is indeed "thin" compared to the size of the Earth!

LESSON LEARNED:  When you tell someone that something is "big" or "small", make sure you establish what you are comparing that something to, i.e. make sure you set the scale for your comparison.

I sometimes tell this story after I admonish people to always ask a scientist how big or small they think "big" or "small" are.  I tell them that I think a "big" gravitational wave, one that we would only expect to see every 10 years or so, would change the length of LIGO's 4 km (2.5 mile) long arms less than 1/1000th the diameter of a proton (10-18 m).   That may be "big" to me now, but to a 5-year-old me something smaller than an atom would be most certainly be considered "small".