Thursday, April 7, 2016

The Source of GW150914: Stellar Mass Black Holes

On September 14th, 2015, LIGO made the first direct detection of gravitational waves.  This event is labeled GW150914 (referring to the year, month, and day of the detection).  The objects that produced the GW150914 were a pair of stellar mass black holes that orbited each other and gradually moved closer and closer together over the course of eons.  The closer together they became, the faster they orbited around each other and the stronger the gravitational waves produced.  LIGO detected the last 0.4 seconds of these stars orbiting until they became so close they merged into a single black hole.

While we saw the death of this paired (binary) system, we didn't get to observe other parts of its life.  Where did these black holes come from?  To answer this question, we need to apply what we know about stellar evolution.


There are several classes of black holes, determined by their mass and how they were formed: stellar mass black holes, intermediate mass black holes, and supermassive black holes.  For stellar mass black holes, they formed when the most massive of stars (more than 15-20 times the mass of our Sun) run out of nuclear fuel and gravity takes over and collapses the star.  For smaller stars, this collapse stops when the pressure from inside the atom (neutron pressure) equals the pressure from the gravitational collapse.  But for these more massive stars, there is no pressure that can stop the collapse and a black hole is formed.  It is in this way stellar mass black holes are the corpses of the most massive stars (but these kinds of black holes are among the least massive).  The newly merged GW150914 black hole now holds the record for the largest stellar mass black hole known.

There are several theories about how this happens... Sometimes this collapse is accompanied by an explosion called a hypernova and is believed to be the source for a kind of gamma-ray burst.  Sometimes the gravity of the collapsing star is so great that all of the matter and light gets sucked into it even if there was a hypernova-like explosion.   


But how did two stellar mass black holes come to be paired together?  The most likely explanation is that they also lived their lives together as a binary star system.  This is very common as it is estimated that about 1 out of 3 stars are in systems of 2 or more stars.  This binary system would likely have formed together and lived their entire lives paired.  The more massive of the 2 stars would have died first since the more massive the star, the faster it burns through its fuel.  Once the nuclear fuel ran out, the more massive star collapsed into a black hole making the system a star/black hole system.  Eventually, the second star would run out of fuel and collapse into a black hole as well making our stellar black hole binary system.  These black holes would orbit for eons before they were close enough to merge and produce the gravitational waves LIGO detected.

In a recent paper (see reference below or read it here), simulations of millions of stars with different material compositions (specifically metalicity which, to an astronomer, is anything that isn't hydrogen or helium; the Sun is 2% 'metal') were simulated and some produced similar outcomes to what we observed.  What was found was that there were similar characteristics for the stars the went on to resemble the GW150914 binary system and this gives us estimates on the time needed for each stage in the system's evolution from birth to the gravitational-wave-generating merger.

The two stars were born about 2 billion years after the Big Bang and were each somewhere between 40 to 100 times the mass of our Sun.  These low metalicity stars (only about 0.06% 'metal') orbit each other as stars for about 4 million years until the more massive one collapses into a black hole.  The now star-black hole system orbit each other for another 1.5 million years until the other star collapses into a black hole.  Both of these stars were massive enough that there wouldn't have been a hypernova-like explosion for either of them; any material ejected would have fallen back into the black hole.  Our new black hole binary system, which is just the corpses of once very massive stars, now go on to orbit each other for over 10 billion years - that is 1000 times longer than the either star was a alive.  At the end of that time, they merge and produce the gravitational waves that LIGO detected 1.3 billion years later when they arrived at Earth.


The short answer: nothing.  This new single black hole is spinning (it is the first detection of a Kerr rotating black hole) but its shape and center of mass are not moving in a way that will ever produce gravitational waves again.   Gravitational waves are also the only way this system would ever have been detected since there wasn't any matter (like dust or gas) to fall into the black holes and generate X-rays.  We will never be able to observe this black hole again.

Of course, there are extremely unlikely events like another black hole flying by and crashing into it...  That may make new gravitational waves for us to see (but I wouldn't hold my breath).


K. Belczynski, D. Holz, T. Bulik, R. O'Shaughnessy, "The origin and evolution of LIGO's first gravitational-wave source" arXive e-Print: 1602.04531 (2016).

Thursday, February 11, 2016

LIGO Makes the First Direct Detection of Gravitational Waves

On morning of 14 September 2015 at almost 4:51 am in Louisiana (09:50:45 UTC) the LIGO detectors in Livingston, LA and Hanford, WA detected a gravitational-wave signal we've labeled GW150914 (based on the date).  The online (near real-time) data analyses alerted scientists about 3 minutes later that there was something of substantial interest in the data.  While vetting this signal (that only lasted about a half of a second) took a substantial amount of time, it opened the new field of gravitational-wave astronomy.  We had not only made the first direct detection of gravitational waves but we also made the first direct detection of a black hole binary (pair) system and proved that these kinds of systems really do exist (it was contentious because the formation of one of the black holes was expected to have destroyed the star that would have made its partner).

At the time of the posting of this blog, the press conference making the announcement is going on and I am working the satellite event being held at the Livingston Observatory.  I will be sure to update this post with the link to the recording or the announcement later (update: see the bottom of this post).  There is too much to talk about in just this post, so I am going to keep this to the basics: what did we see and what does it mean?  I will be doing a series of posts about what we did to make sure that this is a real gravitational wave, the astrophysics of the source, how we detected it, the creation of black holes and why finding a pair like we did is important to astronomy.

Update: Read the Physical Review Letters journal article here.


This gravitational-wave detection was seen as a common signal between the two LIGO sites:

This image shows the data (top row), signal (middle row), and what's left over after the signal is subtracted from the data (bottom row).  Detailed discussion on each image is provided below.

What you see here is a series of images (above and in detail below) that picks apart the signal that was detected.  In the left column is information focusing on the Hanford Observatory and on the right the Livingston Observatory.


The vertical (Y-axis) units are strain with a scale of 10-21.

In the top row is the signal that was seen.  However, this is not the raw data as it was collected.  What you see here is data that has been filtered to 1) reduce noise and 2) to include only frequency components that are around the frequency range of the signal itself.  The red graph on the left is the signal as seen at Hanford and on the left the blue trace is as seen at Livingston.  For comparison, the light red line under the blue Livingston line is the Hanford signal that has been shifted in time to account for the travel time between detectors and flipped (multiplied by -1) to match the orientation of the arms (the arms of each site have a opposite orientation compared to each other so the positive signal in one detector will be negative in the other).  The common signal can be seen with the noise in this comparison.


The vertical (Y-axis) units are strain with a scale of 10-21.

These plots compare the signal predicted by numerical relativity (which are results of computer simulations where the predictions of general relativity cannot be solved by in explicit mathematical expressions) for a pair of black holes with one mass 36 times the mass of our Sun and the other 29 times.  (The red line in the left plot for Hanford and the blue line on the right for Livingston.)  Beneath each of these lines are grey shadowed areas that show the signal as detected from actual LIGO data with two different independent data analysis methods (wavelet and template).  Here again, we can see that the predictions and observations match well.


The vertical (Y-axis) units are strain with a scale of 10-21.

These are plots of residual signals which are the noise that this left behind when the gravitational-wave signal is removed.  Seeing that there is no pattern left in these plots supports that what was seen was a real common signal - a real gravitational wave (this is necessary for a gravitational wave detection but not sufficient - the extra investigations performed will be the subject of a future post).


A powerful tool in signal analysis is breaking up a signal into its frequency components in a graph called a spectrogram.  It allows us to see how much of a signal is made up different frequencies at different times.  If you can hear, then you do this everyday.  It is how you are able to pick apart the sound of a tuba from the sound of a flute when you listen to a symphony.  Both are playing at the same time, but you don't confuse their sounds as coming from anything else.

Below is the spectrogram of this gravitational wave detection:

The horizontal (X-axis) is the progression of time (like above) and the vertical (Y-axis) is showing the contribution of each possible frequency.  The more yellow at a frequency, the stronger that frequency's contribution to the signal at that time.  Our gravitational wave starts at a low frequency (about 35 Hz) and increases to higher frequency (about 250 Hz) near the end of the signal.  This is similar to a signal a slide whistle increasing tone would produce.


As I've mentioned in a previous post, the frequencies of gravitational waves that LIGO is sensitive to would be audible if they were sound waves (which they aren't).  Because of this, we can make them into sound waves by putting the signal through a speaker.  So we did!

Because the starting frequency of the gravitational wave is very low, it is difficult to hear.  The frequency is audible, but at that low of a frequency we tend to feel the sound vibration more than we hear it.  So unless you have a truly great subwoofer, you will probably only hear the end "whoop" of the signal.  In order to make the entire signal more audible, we shifted all of the frequencies up in the above sound up so you can hear the whole thing.  This is not unlike the false-color images made in astronomy for light that our eyes cannot see.

Now that you've heard the detected gravitational wave, you can see that when the tone of it becomes higher toward the end of the signal, the frequency in the spectrogram also goes up.


Because the two LIGO detectors were the only detectors operating at the time of the event (Virgo in Italy is finishing their advanced detector upgrades and KAGRA in Japan is under construction with similar advanced instrumentation) it isn't easy to state precisely where the signal came from.  We can narrow it down to an area on the sky based on how long it took the gravitational wave to travel between the two LIGO detectors, and other factors like the strength of the signal in each detector (there is a slightly different response for each detector for different sky locations).  The most probable location is in the southern hemisphere around the constellations Volans and Carina:

The colored area on this map shows the most probable source of the detected gravitational wave where red is more likely than purple.  The location is shown against a map of the night sky centered on the Milky Way galaxy with constellations outlined.
[Credits: NASA Deep Star Maps (Visualization Credits, Ernie Wright (USRA): Lead Animator, Tom Bridgman (GST): Animator) by NASA/Goddard Space Flight Center Scientific Visualization Studio with constellation figures based on those developed for the IAU by Alan MacRobert of Sky and Telescope magazine (Roger Sinnott and Rick Fienberg), and the source location based on Gravoscope screen grabs (LIGO & Nick Risinger,, all in galactic coordinates. Composition by University of Florida / S. Barke.]


Two different data analysis methods that look at the data in fundamentally different ways not only detected this event, but provided the same results for what the source of it was.  This gravitational wave was made by two stellar mass black holes (these are the remnants of extremely massive stars that have expended their fuel and collapsed under their own gravity).  As quoted above, their masses were about 29 and 36 times the mass of our Sun.  They orbited around each other for hundreds of thousands to millions of years before they come close enough together to start orbiting very quickly (much like an ice skater spins faster as they draw their arms into themselves).  LIGO was only sensitive to the very end of this process right before the two black holes merged into one black hole.  At the end, the stars had a relative velocity of about 1.8x108 m/s, or 60% the speed of light (the universe's "speed limit").  Imagine that...  Two black holes that were each the size of cities but each about 30 times as massive as our Sun whirling around each other at more than half the speed of light!  The animation below shows what it may have looked like to see these black holes merge together.  Note that since they are black holes, no light come from them directly but they do bend the light that is coming from behind them in a process called gravitational lensing:

Based on how strong we know these gravitational waves were at their source as predicted by general relativity and how strong they were once they reached Earth, we estimate that this system is located about 1.3 billion light years (~410 Mpc) away.  That distance is about 10% of the way to the edge of the observable universe!  It also means that the gravitational waves we just detected have been traveling into the universe and toward us for 1.3 billion years.  When these gravitational wave were created the Earth was in the Proterozoic eon of Precambrian time, after when multicellular life developed but before animal life.


Note:  Fast forward to 26:30.  It's just waiting before that. 

Next post: On the formation of stellar mass black hole and why this pair of them are interesting to astronomy...

Wednesday, November 4, 2015

How We Monitor Data Collection with Advanced LIGO

The first Advanced LIGO observing run (O1) started in mid-September and will end in mid-January.  Today I want to tell you about how we collect our data.  On the surface this is obvious: with computers and sensitive electronics.  But how do we keep the detector working so that we can collect data and how do we know that our data is good?

The LIGO Livingston control room on 3 November 2015 (during O1).


The most important step in collecting data is that the detector is working.  This is the primary job responsibility of the roughly 10 operators who work at the site.  There are 3 10-hour shifts a day, each one overlapping with the previous operator's shift by 2 hours so that the incoming operator can be brought up to speed on any issues that may be ongoing.  Since O1 will last into mid-January, that means that there will be at least the operator in the control room every night, weekend, and holiday - even during Thanksgiving dinner, Christmas morning, and New Years at midnight!

During their shift, they monitor various things like the power of the laser, local vibrations, and a multitude of other readings from all over the detector that tend to drift over time.  This work is mainly to prevent a fault in one of the systems that would interrupt data collection.  When everything is working the way it is supposed to, this part of their job can be boring - and we love boring days and nights. 

Excitement happens when we are no longer able to keep the light bouncing back and forth between the mirrors (we call this "breaking lock").  The operator's job now is to respond by discovering if the lock was lost due to an environmental issue we can't control (like an earthquake anywhere on the planet) or due to an detector issue.  If there is a malfunction in the detector, the operator identifies what subsystem caused the problem and then uses their training to fix it and get the detector up and running again.  Through my conversations with them, one of the harder parts of their job is identifying which part of the detector isn't working properly since there are so many subsystems that need to work all at the same time for us to be able to collect data.


During the Initial LIGO science runs, there were always 2 people in the control room: the operator and the "scimon" (short for science monitor).  The scimon's job was to ensure the quality of the data that was being collected and give feedback to the operator.  Scimons came from institutions across the country who would usually spend a week or two at the observatory before returning to their home.  This meant that there were a lot of people passing though the observatory (which isn't bad) and by the time they really got comfortable in their job it was time for them to go home (this isn't good). 

We are doing the science monitoring differently for Advanced LIGO: we have longer-term (several months) visiting scientists (LSC Fellows) working on site to monitor the data as it is collected and we have data quality scientists (we call them "DQ Shifters") who remotely monitor the properties of the data for a period of 3-4 days.


These scientists are on-site to monitor the data as it is collected and they also each have a project related to improving the instrument.  There is almost always a fellow on-site except for the earliest hours of the day (they are not as necessary as the operator and their instrument research is best done when other scientists are also around).  The fellows work with the operators to identify subsystems that may be causing issues and they work to resolve them.  Basically, these are the Advanced LIGO version of the scimon but with the benefit of having the visiting scientist being able to apply what they learn while on site.


The DQ shifter is a scientist who monitors the quality of the data that has already been taken (within about a day or so).  Sometimes, patterns only become evident after a significant amount of data has been collected.  Because this work is not expected to have immediate feedback to the operators and fellows, this work can be done remotely.  We have created automated web pages that have all the plots needed to look at how the different parts of the detector are working.  There are about 40 or so of us (including me) who have been trained on how to interpret all of the graphs that appear on these pages and what specific things we should be watching for.  We communicate with the fellows at the site we are monitoring on a daily basis so that they can use the feedback to improve the quality of the data.  When our shift is done (we usually cover 3-4 days in a shift), we document our findings, report to the data quality group who specializes in studying collected data, and we enter an entry in the detector log with a summary of our shift.

Summary pages used by DQ Shifters to evaluate the quality of data already taken.  These plots specifically show how the ground was moving in different frequency bands throughout the day on 2 November 2015.

Friday, July 31, 2015

First Science Data With Advanced LIGO is Near!

It has been a very exciting time for Advanced LIGO recently.  A few weeks ago we completed a test run of the instrument to identify any remaining bugs in the instrument or other stability issues.  The commissioners (instrumental scientists who work on making LIGO more sensitive) have been busy adjusting various settings in a multitude of subsystems to increase our sensitivity to gravitational waves.  We are continuously learning more about how all of these subsystems react to one another and to the environment.  And learning is never without its own pains.  Some bugs have been bigger than others. We've had to actually touch the new instrumentation - meaning we had to seal off the chamber the part was in, let the air back in (since almost all of the instrument is in a vacuum), fix it, close up the chamber, and pump the air back out.  This is rare but it has happened.  Once the instrument was performing well, that's when we decided to stop tinkering with it and use it like we would if we were looking for gravitational waves.  More subtle issues in stability and other bugs will make themselves apparent only after you use it the way it's meant to be used - all the time.

Installing one of Advanced LIGO's seismic isolation platforms at the Hanford observatory in 2013.


These test runs are called engineering runs.  We abbreviate them ER followed by the number of the run.  The last one was called ER8.  I've already talked about the first one (ER1) back when almost everything was being simulated since the installation of the instruments was just getting off the ground.  The purpose of those early engineering runs was to test out the ability of our data analysis systems to handle the large amount of data we will collect.  As parts of aLIGO were installed, we replaced the simulated data from that component with real data.  ER8 was our first test of all of the instrument without anything being simulated.  While the purpose of this data is to test the stability of the whole system and to find other small bugs, we are still running all of our data analysis methods over the collected data.  We don't expect to find a gravitational wave in this data, but if we have compelling reason to believe that we really did see something we will certainly pursue it as a real detection.  Don't get too excited, though, since there are no indications that we collected a gravitational wave.


What is really exciting is that we are preparing to make that first detection.  We don't really expect to detect a gravitational wave with our first science data (which will be called O1 - observation 1) with aLIGO but it is not as improbable as it was with Initial LIGO.  We are talking about what we learned from the blind injection in our last iLIGO data set (otherwise known as the "Big Dog" event) and what our detection validation should entail.  We are talking about writing the paper that we will publish announcing the first detection and its details.  We are even talking about how we will engage the public with this announcement.  Don't misunderstand me - we have not seen anything yet, but we are preparing ourselves for the possibility of detection.


You really don't have any idea how exciting this is especially for those of us who have been around a while (I have been working on LIGO since starting grad school in 1999 and I'm a youngster).  I have been working on this project that is so much bigger than myself since before we took our first data with Initial LIGO.  I remember when the collaboration was a couple hundred scientists (there are now almost 1000 of us).  I remember when we analyzed our first data and debated how to interpret our detection candidates when we almost sure that everything we had was noise (i.e. garbage).  Now we are talking about confidently making a detection, and doing astronomy with it.  This is the dawn of a new age in astronomy and I'm proud to be here to see it.

Distance in parsecs (1 pc = 3.26 light years) Initial LIGO was able to detect its standard source of 2 neutron stars orbiting each other just before they merge into one body.  (Read more here.)
aLIGO wil be able to "see" up to 200 Mpc (about 650 million light years).
Remember, we don't expect a detection, but it is possible.  To give you an idea of how possible, once we have aLIGO working at the sensitivity it was designed to work at, it will observe as much of the universe in several hours as Initial LIGO did in an entire year.  We won't be at design sensitivity for O1, but we can already detect our standard source 4 times farther away than we could on our best days with Initial LIGO.

An image of light that was filtered out of the laser before entering the LIGO detector.  Bend your neck to the right and you should be able to see a smiley face.  This is just a chance configuration and has no significance, but we thought it was cool.

Monday, June 1, 2015

Advanced LIGO is Here!

I've been away from all of you for a little over a year due to many factors including teaching new courses, starting new research projects, and more than a few personal reasons.  However, I wanted to let all of you know about the status of Advanced LIGO (spoiler: it's done) and that I will be back to posting on this blog on a regular basis.


On 20 October 2010, Initial LIGO (iLIGO) recorded its last bits of science data [read the blog post here].  At that time, we were taking some of the most sensitive gravitational wave data and we thought we may have recorded a real gravitational wave (it was a fake signal purposefully placed in the data to test our ability to find real gravitational waves, but we didn't know that at the time [you can read all about it here]).  The metaphorical "keys" to the detector were transferred from operations to the aLIGO installation team.

In the nearly 5 years since iLIGO, we've removed all of the old instrumentation, much of which had been designed 15 years ago (remember what cell phones looked like back then? - we've come a long way) and replaced it with newly redesigned instruments.  You won't notice anything different by flying over LIGO (there was not real estate expansion) but we gutted at very intricate and technical instrument and replaced it with more sophisticated hardware.  The details on the upgrades could make a whole series of blog posts, but a few of them included improved seismic (ground vibration) isolation, better ways to hang our mirrors like pendula, a more powerful laser, more massive mirror, better coatings on the mirrors, and new ways to reuse laser light to increase the laser power in the the arms.  All of this will combine to make aLIGO over 10 times as sensitive as it was before allowing us to observe 1000 times more of the universe than with the original observations we made.

The illustration above shows the anticipated "reach" of Advanced LIGO (the purple sphere) compared to Initial LIGO (the orange sphere).  Each small dot in the figure represents a galaxy.  Since the volume of space that the instrument can see grows as the cube of the distance, this means that the event rates will be more than 1,000 times greater.  Advanced LIGO will equal the 1-yr integrated observation time of Initial LIGO in roughly 3 hours. (Galaxy map credit: R. Powell,

On 19 May 2015, aLIGO was dedicated at the Hanford, WA observatory (since I am at the Livingston, LA site and generally unimportant, I missed out).  The "keys" are now back with the operations team at both sites (the Livingston site was scheduled to be 'done' before Hanford and has been 'working' for several months now).  Why did I put done and working in quote in my parenthetical comment?  Well, now is the time for commissioning.  The detector can turn on and operate as in interferometer but all of the new components aren't yet optimized to work together resulting in the detector being less sensitive than it was designed to be.  The work that is currently gong in with the detector is commissioning work that seeks to work on individual subsystems so that the detector works better as a whole.  In short, this is our version of tuning up our car.

Break time at the Advanced LIGO dedication at the LIGO Hanford Observatory on 19 May 2015.  [Source: LIGO Scientific Collaboration's Facebook page]

Even though neither detector is working at the sensitivity it was designed to, we are regularly setting sensitivity records when we do turn on the detector to test the commissioning work.  One of the ways we measure our sensitivity is to determine the farthest distance away a standard source of gravitational waves could be for us to just be able to detect it.  The standard source we use is two neutron stars orbiting each other and merging into one.  (We picked this because it is a simple system were we can predict how big the gravitational waves will be and what shape the waves will have.)  We call this the inspiral range.  Below is the insprial range for each aLIGO detector (Livingston is the blue squares line and Hanford is the red dots line) given the number of days since the aLIGO installation was declared complete (there are more data points for Livingston since we were scheduled to be done a little before Hanford).

The distance into the universe we would be able to detect a gravitational wave from our reference source of two neutron stars orbiting each other and merging into one.  [Source: Talk given by David Shoemaker at the aLIGO Dedication on 19 May 2015]

Our best data with iLIGO was able to detect out to about 20 Mpc (a little over 65 million light years away).  Currently, the Livingston's inspiral range is at 65 Mpc (212 million light years) and Hanford's is at 57 Mpc (almost 186 million light years).  So, even though we are still commissioning the detectors, we are already gathering the most sensitive gravitational-wave data ever!


Cristina Torres

I lost a very good friend a few moths ago.  Cristina and I were both postdocs at LIGO Livingston until 2012 when she took a position at the University of Texas at Brownsville as a professor.  We shared a passion for engaging others in our science, but she always had an openness to others that I have admired.  She was a better friend to me than I ever was to her, but if she was here to read this she would argue with me since she did exactly that in one of our last emails. 

The last time I saw her in person was when I was at UT Brownsville earlier this year to speak about work/life balance, which I don't really have figured out, at a regional Conference for Undergraduate Women in Physics (there is a beautiful tribute to her at the bottom of this page).  She was so stressed since much of the local organization and logistics was on her shoulders but the meeting went very well!  If I had any idea that I wouldn't be seeing her again, I would have made more of an effort to spend time with her (instead of just trying to stay out of her hair).

This is a picture of Cristina with a prototype of the new mirror suspension system at LIGO Livingston in 2012.  We use this display to show visitors some of the upgrades that they aren't able to see inside of aLIGO. 

Until again, Cristina...

Friday, March 21, 2014

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


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...


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.)


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.


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. 


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

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


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!

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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!


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."


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).


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!