Wednesday, June 15, 2016

Merry Christmas, LIGO: Another Gravitational Wave!

WE DETECTED ANOTHER GRAVITATIONAL WAVE!

On the evening of Christmas day 2015, at 9:38 pm CST (3:38 am UTC) at the LIGO Livingston Observatory in Louisiana, another gravitational wave signal was recorded.  1.1 ms later, the LIGO Hanford Observatory in Washington state also picked up the same signal.  70 seconds later, the supercomputer that runs analyses on the near real-time data noticed that there was something special in the data and sent out emails and text messages that some of us affectionately call the "Bat Signal".  This goes out to scientists primarily to summon those who evaluate candidate gravitational wave events to determine if this event should be shared with traditional astronomers (i.e. ones with telescopes).  I am on the list because I am interested in keeping up on the latest results.  I remember exactly where I was: I was in my room at my mother's house outside of Pittsburgh changing clothes after getting back from visiting the in-laws (who live within a few miles of my family's home) for Christmas.  I looked at the event record and saw that this was an extraordinary candidate gravitational wave in that its statistical significance was high but the signal wasn't as obvious in graphs as the first detection in September was.

It was decided to send out the location of the possible detection to traditional astronomers and the emails started flying discussing the evidence that this was a true detection.  It was determined that the preliminary information on the signal warranted starting the detection checklist - the large-scale investigations that try to disprove that the signal is real.  Only after a candidate passes every test and has a high statistical significance is it accepted as a detection.  The same checklist that was applied to the first detection, labeled GW150914, was applied to this candidate as well.  Once this Christmas detection was verified, it was labeled GW151226 (the number reflects the UTC date that the gravitational wave was discovered) although we had nicknamed it the "Boxing Day Event" before the verification.

(Below I will often refer to GW150914 as the "first detection" and GW151226 as the "new detection".)

Read the paper on the detection here.


THE SIGNAL & THE SOURCE

The signal is similar to the first detected gravitational wave (GW150914).  We call this kind of signal a "chirp" because initially it has a low frequency which increases over time as does its amplitude.  You've heard signals like this before if you've ever hear a slide whistle increasing in tone.  The increase in tone reflects the increase in frequency and the loudness of the whistle represents the amplitude.  The signal we detected starts at about 35 Hz (close to the frequency of the sound made by the second black key from the left on the piano) and reaches its highest frequency at about 450 Hz (very close to the A above middle C if you convert this signal into sound).

Graph of the 1-second signal of GW151226.  The red line is the prediction of what a gravitational wave from a 14.2 and 7.5 solar mass black hole merger would look like and the grey area around it is the signal that LIGO recovered from its data.  The zoomed in portions allow you to get a better look at hour the prediction (in red) and the actual signal (in grey) compare.  At the end of this signal, the frequency and amplitude both go up.  The two black holes merge at the point where the amplitude of the signal is the highest (seen in the zoomed data to the far right).

The plot above shows what we detected in our data compared to the predictions of a pair of black holes orbiting each other and merging into one.  So this is similar to the last detection in that this is also a pair of stellar-mass black holes (formed from the death of extremely massive stars) but different because the masses of these new black holes are less than the first detection.  Here, our newly detected black holes are 14.2 and 7.5 solar masses where our last detection was 36.2 and 29.1 solar masses.  That makes this signal weaker than the last (the peak amplitude of this new signal is about 1/3 that of the first detection) but we are able to observe more orbits of the system here.  We see about 27 orbits of these new black holes (corresponding to the 55 cycles of the gravitational wave we see in the figure) where we only saw about 5 orbits (or 10 cycles) in the first detection.  It is interesting to note that lower mass black hole pairs will merge at higher frequencies than higher mass black holes.  This means that the signal will stay in LIGO's most sensitive frequencies longer and that is reflected in what we see here.  This new detection's signal is about 1 second long while the first detection is less than a half second long.

So, what did this new detection look and sound like?  As far as what it looked like, there was no light that we are aware of that was produced from this system.  But we can visualize the black holes as they orbit around each other and track the corresponding progression through the signal to the merger.  [Credit: SXS Collaboration/www.black-holes.org]:


video


We can also "listen" to gravitational waves by taking the signal, and converting it into sound through your speakers.  Below is a comparison of what the new detection "sounded" like compared to the first detection.  The actual "sounds" are quite low in tone so that they sound more like thumps.  We also have shifted the sounds up to a higher tone so that you can hear more of the detail in the signals.  That will play after the original lower tones.  The background graph shows how the frequency changes on the vertical axis (you will see that it increases for both signals) as time progresses on the horizontal axis:


video


The next question is where in the sky are these black holes?  We primarily determine this using the delay in detection time between the two detectors.  When the delay is large, there is a smaller area in the shape of a ring on the sky where the gravitational wave could have come from.  The detection delay for the new detection is much shorter than the first detection, so our uncertainty is going to be larger.  Below is an illustration of the areas on the sky where the new detection (the area to the left) and the first detection (the area to the right) are likely to have come from.  Note that for the new detection on the left, there is another similar area on the opposite side of the sky that cannot be seen in this image.


The location of both the new GW151226 detection (on the left) and the first detection, GW150914 (on the right).  These are pictured on a star map (you can see the center part of the Milky Way galaxy on the left and extending right).  There is another similar area for the new detection on the opposite side of the sky (not pictured here).  The outer purple area is where we are 90% confident where the sources are located.  The inner circles each have decreasing certainty.


We will be better able to determine the location of a gravitational wave source on the sky when we have more than two detectors in operation.  Fortunately, Advanced Virgo has completed their upgrades and is currently testing their new detector.  LIGO's next observing run is expected in the 4th quarter of this year and Advanced Virgo will likely join the search before the completion of that run.  When we detect more gravitational waves (which we expect since we will be even more sensitive than we were for the two detections we have already made and the run will be longer in duration) together with Virgo, we will know even more about what it is that we are seeing.

This is an exciting time to be a scientist!

Read the official LIGO "Science Summary" on this new detection, GW151226.

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.


STELLAR MASS BLACK HOLES ARE CORPSES

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.   


THE EVOLUTION OF THE GW150914 SYSTEM

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.


WHAT WILL HAPPEN NOW?

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).


Reference:

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.


THE SIGNAL

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.

TOP ROW:

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.

MIDDLE ROW:

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.

BOTTOM ROW:

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).


THE SPECTROGRAM

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.


WHAT WOULD THIS SOUND LIKE?

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!

video

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.

video


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.


WHERE DID THE SIGNAL COME FROM?

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, skysurvey.org), all in galactic coordinates. Composition by University of Florida / S. Barke.]







 
WHAT MADE THIS GRAVITATIONAL WAVE?

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:

video

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.

PRESS CONFERENCE RECORDING

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).

OPERATORS

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.

SCIENTISTS

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.

SCIENTISTS: LSC FELLOWS

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.

SCIENTISTS: DQ SHIFTERS

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.

ENGINEERING RUNS

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.

OBSERVING RUNS

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.

DETECTION IS POSSIBLE...

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.

ADVANCED LIGO

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, www.atlasoftheuniverse.com.)

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!


IN MEMORY:

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 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.

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: