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The Messenger
http://www.spaceref.com/news/viewpr.html?pid=19142

QUOTE
ST. LOUIS, Mo. -- The quest to detect and study gravitational waves with the NSF-funded Laser Interferometer Gravitational-Wave Observatory (LIGO) is now in the fourth month of its first sustained science run since achieving its promised design sensitivity, project personnel announced at the annual meeting of the American Association for the Advancement of Science (AAAS). ...

Now that the LIGO is sensitive enough to detect changes in distance a mere thousandth the diameter of a proton, Marx adds, the science return should be even greater. Recent results from the Swift satellite pinpointing the location of short gamma-ray bursts (GRBs) have also heightened astronomers' interest in the results from LIGO's current observational run.


That level of sensitivity is, in my opinion, the most incredible technical achievement since the VLA.

The very long gamma ray associated with supernova/hypernova 1996aj should also be of great interest.
Richard Trigaux
So we shall soon know if gravity waves really exist.



Is it necessary for this to wait for the end of the run? Isn't it possible to check the data on the fly, so that an event may be seen as soon as it happens? (Not true if the expected event probability is of one per year, as I heard once).

Also it may be possible to have a background noise spectrum before the end of the run, even if not so accurate.


After reading the article, they expect a direct detection only if "nature is very kind". So perhaps they will announce one... or we still have some years to wait. Otherwise they will get only upper limits on various phenomena.

The LIGO site
The Messenger
QUOTE (Richard Trigaux @ Mar 3 2006, 08:43 AM) *
So we shall soon know if gravity waves really exist.
Is it necessary for this to wait for the end of the run? Isn't it possible to check the data on the fly, so that an event may be seen as soon as it happens? (Not true if the expected event probability is of one per year, as I heard once).

Also it may be possible to have a background noise spectrum before the end of the run, even if not so accurate.
After reading the article, they expect a direct detection only if "nature is very kind". So perhaps they will announce one... or we still have some years to wait. Otherwise they will get only upper limits on various phenomena.

The LIGO site


The data reduction on LIGO takes months, and that is with a lot of number-chrunching on the Einstein at home network. Basically, they have to scrutinize every single bump and grind, and filtering out every Earth vibration is a daunting task. I would assume they will be crunching this eighteen month run on the fly, and if anything definitive happens, we will know as soon as they are certain.

The current constraints on gravity waves are lower than most theorists anticipated, but not lower than pessimistic estimates of what the gravity parameters should be for neutron star, black hole mergers and such. The "if nature is kind" clause is pessimistic, and some (most) theorists involved in the project expect positive results if the current sensitivities can be maintained for the full 18 month run.
Richard Trigaux
QUOTE (The Messenger @ Mar 3 2006, 06:02 PM) *
The data reduction on LIGO takes months, and that is with a lot of number-chrunching on the Einstein at home network. Basically, they have to scrutinize every single bump and grind, and filtering out every Earth vibration is a daunting task. I would assume they will be crunching this eighteen month run on the fly, and if anything definitive happens, we will know as soon as they are certain.

The current constraints on gravity waves are lower than most theorists anticipated, but not lower than pessimistic estimates of what the gravity parameters should be for neutron star, black hole mergers and such. The "if nature is kind" clause is pessimistic, and some (most) theorists involved in the project expect positive results if the current sensitivities can be maintained for the full 18 month run.



Yes, if they don't need things such as large windows or long-term averaging, they can do on the fly, the only delay being the calculation time.

Only if they were interested in very low frequencies they would need averaging on the whole run. If there is for instance something like a cosmological background oscillating at periods of days or more, we can know it only at the end of the run. But if there is something like a supernova or a black hole spiraling, they would recognize it immediately, or at least after the calculation time.

I can imagine the task -recognizing predictable patterns into an overwheelming noise- as I was somewhat involved in this when I was workig. This is also the way SETI works. (a spin-off of SETI!). But what if an UNEXPECTED signal comes? Will them just cull as noise all what is unexpected?
The Messenger
http://einstein.phys.uwm.edu/forum_thread.php?id=4013

QUOTE
Last modified: 3 Apr 2006 11:11:13 UTC
The S4 run is about to cross the 50% line!!! :-)


...
GO E@H!!
edstrick
LIGO can potentially detect 1) predicted types of sources and 2) unpredicted types of sources. It cannot detect some types of sources that are predicted to have most of their energy at wavelengths much longer, frequencies much lower, than it's high frequency sensativity range.

The proposed LISA space-interferometer gravity wave mission would search for much lower frequency sources and is predicted to observe many to the point of having considerable confusion sorting out things like white-dwarf binaries and cosmic backgrounds.

The problem with LIGO, Mark 1, is it's sensativity is so low that the PREDICTED frequency of detectable events of PREDICTED type, the classic being binary-neutron star "in-spirals", is considerably lower than 1 per year. So they're searching down in the noise level for barely detectible, if at all, events.

We COULD get locky... and have a predicted type of source do it's thing so close there's an obvious signature well above the noise...but don't count on it.
We COULD get luckier, and have predicted type of events happen much more often or have unpredicted types of events happen often enough and close enough they also stick out above the nose. Again... don't count on it.

LIGO, Mark 2... The second generation detector system, is intended to have high enough sensativity so that there are multiple events per year that are clearly detected.
Richard Trigaux
QUOTE (edstrick @ Apr 4 2006, 09:31 AM) *
We COULD get locky... and have a predicted type of source do it's thing so close there's an obvious signature well above the noise...but don't count on it.
We COULD get luckier, and have predicted type of events happen much more often or have unpredicted types of events happen often enough and close enough they also stick out above the nose. Again... don't count on it.


Translation: LIGO may detect nothing in all the run. But if so, it would not be an evidence that gravity waves don't exist.


In SETI (I alway compare to SETI, as basically it is the same problem: sort out signals from noise) they also try to find recognizable patterns, such as pulses, multiple pulses, or single frequencies shifting (with the motion of the emitter). The problem they have to face is that there is so much records that, statistically, any pattern you search for is likely to appear from mere random noise. So you have to consider the overall probability of a pattern match in all the data, and it can be much higher than expected. Despites this they do have some alarms which occurence at random are very low (one billionth or less). If they don't claim such alarms as positive detection, it is only because they have further tests to accept these alarms, for instance that they happen several times on the same place.

With LIGO there is only one set of data (there is no direction) but still a long time and many frequencies. So if we look in depth into the data, what is likely to happen is something for instance like a whistle of shifting frequencies which probability to appear from random noise is equal or higher to the probability of coming from a (far) black hole spiraling. Such a result would "encourage to build a more sensitive experiment", but I would prefer a clear result, a strong enough event which probability to appear from noise is very low. But we still need to be lucky...


Of course if a clear signal is detected, this will much more encourage to build more sensitive detectors, as gravitationnal waves would be a new and completelly different window on the universe, where we directly see the movements of large/dense bodies.
The Messenger
The data being reduced on Einstein at Home computers (right now) is from science run 4 (S4). It may be necessary to process all of the data before a determination can be made as to whether an event has been witnessed. I know, for example, that about a third of the way through run three, they determined new baseline criteria (based upon the analsysis of noise levels), and restarted crunching the numbers.

Richard's analysis is consistent with what I have read elsewhere - the odds witnessing a gravity wave in this data are small.

Their is much more optimism about the current run (S5), if the current level of sensitive can be maintained. In fact a number of scientist near the project have stated that a null result through ~2010 should be considered a 'successful' non-detection, and GR theory would have to be slated for revision.
The Messenger
http://einstein.phys.uwm.edu/forum_thread.php?id=4092

QUOTE (Ben Owen)
In LIGO land we've all been running full steam for a while, but it's time for an update.

S5 (the fifth science data run) has been going since early November and is scheduled to last about a year. It was certified (I think by the National Science Board?) as having reached the initial design goal, so we're officially in business.

Roughly speaking, S5 data is about twice as good as S4. That is, the strain noise is half what it was in S4. But there is room for improvement.

First, the duty cycle for triple coincidence (H1, H2, and L1 interferometers all running) wasn't so hot. The main culprit was L1. Before S4, Livingston was down every day due to logging next door. Improved seismic isolation fixed that, but for the first couple of months of S5 they started adding another building right next to the corner station. It's an education and outreach center, full of all sorts of goodies for the general public. That'll be great, but during the day it meant L1 was down even with the new isolation. The building is done, so we're seeing better duty cycle now.

Also, the noise below 100Hz was actually a bit worse than design. That's been slowly improving as some things were caught, but we still don't know what's causing it. Fair bet it's somehow up-conversion of 1-3Hz seismic noise from traffic, since it correlates pretty strongly with the working day and rush hour. Last time I did shifts in the control room was January, and during the day it was enough that we had to lower the laser power to avoid dropping out of lock constantly. One gravel truck wouldn't do it, but two in rapid succession would nail us. One figure of merit is the range to which we could see a signal from a binary neutron star signal. That was peaking at 12 megaparsecs for H1 back in January and dropping to 10 during the day. Now it's up to almost 14 on a good night.
...
Holder of the Two Leashes
LIGO team is going to update us all tomorrow, and may be making an announcement.

Thursday Feb 11, 2016 LIGO news conference
Holder of the Two Leashes
News conference about to start. In the auditorium, they have posters up showing the merger of two black holes.

EDIT: CONFIRMED - Merger of two black holes observed at both observatories 7 milliseconds apart on September 14, 2015.

EDIT: They are saying that three solar masses of energy was converted to gravitational waves. The black holes were approximately 36 and 29 times the mass of the sun, the resulting black hole was 62 solar masses. The signal LIGO detected lasted half a second. Event occurred 1.3 billon light years away, in the general direction of the Magellanic Clouds (but far beyond them).
Mongo
Archived Livestream of the press conference has been taken down. Here is the Youtube version.
nprev
Three solar masses converted into energy in milliseconds...just have to ponder that for a moment.

In any case, what an astonishing achievement!
Mongo
Numerous papers about the discovery can be found at the LIGO Document Control Center. Of particular interest:

Observation of Gravitational Waves from a Binary Black Hole Merger (914 kB - the Physical Reviews Letters paper)
Properties of the binary black hole merger GW150914 (6.3 MB)
Astrophysical Implications of the Binary Black-Hole Merger GW150914 (666 kB)
Tests of general relativity with GW150914 (942 kB)
hendric
Nice article about the post-doc who was monitoring when the event occurred:

http://www.sciencemag.org/news/2016/02/her...itational-waves
PFK
QUOTE (hendric @ Feb 11 2016, 07:36 PM) *
Nice article about the post-doc who was monitoring when the event occurred:

http://www.sciencemag.org/news/2016/02/her...itational-waves


Great stuff. "He reached only one of the facilities—“Livingston, I think,” he says—but was told all was normal".
If only his words were "Livingston, I presume" smile.gif
Mongo
An interesting possible second binary black hole merger, from GW150914: First results from the search for binary black hole coalescence with Advanced LIGO

QUOTE
The second most significant candidate event in the observation period (referred to as LVT151012) was reported on October 12, 2015 at 09:54:43 UTC with a combined matched filter SNR of 9:6. The search reported a false alarm rate of 1 per 2.3 years and a corresponding false alarm probability of 0.02 for this candidate event. Detector characterization studies have not identified an instrumental or environmental artifact as causing this candidate event. However, its false alarm probability is not sufficiently low to confidently claim this candidate event as a signal. Detailed waveform analysis of this candidate event indicates that it is also a binary black hole merger with source frame masses 23 M Sun and 13 MSun , if it is of astrophysical origin.
TheAnt
Now that a second black hole merger have been detected with high confidence. The idea that such events might be common have surfaced and made some think in radically new lines to explain how such events might happen quite often.
One hypothesis is that black holes and stars in the central part of globular clusters might collect in a 'Mosh pit' - I have a hunch that one of the team must be a metalhead for that expression to be used - indeed is a good term to describe the chaotic conditions and collisions that can occur in such a volume of space where stars and black holes are comparably tightly packed together.
Northwestern univ summary, video and link to the actual paper.
moustifouette
First merger of neutron stars detected and observed both with gravity waves and electromagnetical ones.

ligo press release
JRehling
The neutron star merger is actually bigger news than the black hole mergers, as it just provided the explanation for how half the elements on the periodic table are created, and provided absolute confirmation of how some gamma ray bursts are generated, while establishing with certainty that the speed of gravity and the speed of light are equal. Wow! That's a lot of science from one event!

I'm eager to see more neutron star mergers to provide information on the base rates of such events. And incidentally, I might be able to photograph one of these in the northern skies someday.
Explorer1
Incredible find!
I was under the impression that supernovas also create the elements heavier than iron. Is that origin theory still possible, or is it a mix of both?
TheAnt
QUOTE (Explorer1 @ Oct 19 2017, 02:43 AM) *
Incredible find!
I was under the impression that supernovas also create the elements heavier than iron. Is that origin theory still possible, or is it a mix of both?


A supernova should indeed produce some heavy elements if the theory is correct for them.

If I have understood this right, and I'm no physicist, the amounts of elements heavier than iron don't quite add up if only supernova explosions were the source.
For example the amount of the most heavy elements would be very small. Lets say the mass of Luna, our moon from one supernova.
When two neutron stars collide, they spew out 100 if not 1000's of times more material.
The part from the surface of the neutron star would be iron and elements from that medium heavy part of the periodic table, the other material will be neutronium.
Soon ˝ of the neutrons decay to protons capture electrons and create gold, uranium and the very heavy, and briefly existing, elements on row 7 of the periodic table.
nogal
QUOTE (moustifouette @ Oct 17 2017, 08:19 AM) *
First merger of neutron stars detected and observed both with gravity waves and electromagnetical ones.

ligo press release


See also INTEGRAL sees blast traveling with gravitational waves
fredk
QUOTE (JRehling @ Oct 18 2017, 11:25 PM) *
The neutron star merger is actually bigger news than the black hole mergers

A bit of an apples and oranges thing, perhaps. The BH mergers were extremely cool since we had direct observations of the "strong-field" regime of gravity, where the spacetime undergoes order-unity convolutions. That's many many many orders of magnitude stronger than anything we'd seen before, and Einstein's general relativity (GR) passed with flying colours.

The equality of electromagnetic and gravitational wave speeds is extremely cool, too. It's meant that many large classes of models for dark energy or modified gravity (extensions of GR) have been ruled out overnight.
Floyd
It is amazing that a lid was successfully placed on the news and the findings were successfully announcement at one time. An article in Science stated that the huge teams of physicists were unprepared for the chaos of astronomers in small competing groups---but they came together on one giant paper submitted to The Astrophysical Letters with 4600 authors--one third of the astronomy community. Wow, talk of big science.
TheAnt
QUOTE (Floyd @ Oct 22 2017, 04:24 PM) *
.....one third of the astronomy community. Wow, talk of big science.


They needed time to write, and embargoed each other (the cloak and dagger threat works fine in my field - and it's nothing regarding space at all) but it worked fine for them also.

Big science indeed, on the bottom of this page, and unusual for Universe today - a bibliographical list with 96 papers on arXiv.

@fredk: Yes you're right there that most MOND ideas have been thrown out the window, and also several models for Dark energy. I think the more outlandish idea of branes made it trough this unscathed.
Holder of the Two Leashes
LIGO and VIRGO are about to begin a new year long science run, with significantly increased sensitivity.

LIGO's last observing run went from November 30, 2016 to August 25, 2017. It has been down since then in order to install improved detectors.

LINK to article at LIGO website

They won't be sitting on detections for months before announcing them anymore. Anything that looks like gravitational wave event will be posted on a public site in near real time.

User guide for public announcements

Good hunting!
Holder of the Two Leashes
Latest results are the second detected binary neutron star merger, and just a day later a possible (and never before seen) black hole - neutron star merger.

Article at CalTech
antoniseb
S190521g was a binary black hole very far away, probably about 3300 megaparsecs (note: the microwave background is about 4200 megaparsecs).
I was not aware that the new upgrades allowed detecting events that far back.
fredk
The CMB is at around 14 comoving Gpc, not 4.2 (see, eg, this calculator). Also, without an optical counterpart there will be large uncertainties on the distance to a merger.
antoniseb
Interesting. Is the GraceDb (https://gracedb.ligo.org/superevents/S190521g/view/) reporting distance in comoving megaparsecs? That seems odd, but would explain why this event was observed at all.
Can you point me to any place where they say or imply that they are reporting in comoving Mpc? Thanks in advance!

fredk
Do you mean comoving vs proper distances? It's standard in cosmology to use comoving distances, since it avoids the problem of separations being smaller in the past. For close distances (redshift z << 1) proper distances are well defined and are very close to comoving ones.
Lucas
It appears that they are using luminosity distances (see http://www.astro.ucla.edu/~wright/cosmo_02.htm#DL for definition) based on https://lscsoft.docs.ligo.org/ligo.skymap/l...p/bayestar.html (look at min_distance parameter) and https://emfollow.docs.ligo.org/userguide/glossary.html (look at “burst range”). In these units the CMB is at ~15000 Gpc.
fredk
Yes, the observed gravitational wave amplitude (and frequency and its rate of change) gives the luminosity distance (once you've got a handle on the polarization), but at this level of accuracy that will agree with any other distance measure. Units, though, are comoving. The CMB is at just under 14 comoving Gpc, not 15 000 - see, eg, sect 3.4 of Planck Inflation. So at very crudely a Gpc, this merger is considerably closer.
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