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 0.2 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? A 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).
Showing posts with label first detection. Show all posts
Showing posts with label first detection. Show all posts
Friday, April 8, 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:
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:
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:
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:
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!
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.
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:
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:
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...
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!
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.
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:
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...
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