A gift for Jerry: a cosmology article after his heart - Page 2

William · #31 (**permalink**) 04-September-2007, 03:39 AM

Fortunate,

The following paper, published January 2007, I believe is the most recent, formal update of the current GRW interferometer status and results. It states that the current operating interferometers have reached sufficient sensitivity to detect the GRW which the community believes theoretically should be emitted by specific galactic events. (The problem is no longer sensitivity of detector, but rather frequency or occurrence of event.)

"Search for gravitational-wave bursts in LIGO data from the fourth science run" by approx. 260 authors.

http://lanl.arxiv.org/pdf/0704.0943

Quote:

Introduction:

Large interferometers are now being used to search for gravitational waves with sufficient sensitivity to be able to detect signals from distant astrophysical sources. At present, the three detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) project [1] have achieved strain sensitivities consistent with their design goals, while the GEO 600 [2] and Virgo [3] detectors are in the process of being commissioned and are expected to reach comparable sensitivities.

See paper for details concerning number of significant event occurrence. Note black hole merger theoretically produces a prodigious amount of GRW.

Quote:

We also provide rough estimates of the distances at which representative supernova and binary black hole merger signals could be detected with 50% efficiency by this analysis.

No GRW detected to date.

Quote:

With no unshifted triggers in the final sample, we place an upper limit on the mean rate of gravitational-wave events that would be detected reliably (i.e., with efficiency near unity) by this analysis pipeline. Since the background estimate is small and is subject to some systematic uncertainties, we simply take it to be zero for purposes of calculating the rate limit; this makes the rate limit conservative. With 15.5 days of observation time, the one-sided frequentist upper limit on the rate at 90% confidence level is −ln (0.1)/T = 2.303/(15.5 days) = 0.15 per day. For comparison, the S2 search [17] arrived at an upper limit of 0.26 per day.

Quote:

The GEO 600 interferometer has joined the S5 run for full-time observing in May 2006, and we look forward to the time when VIRGO begins operating with sensitivity comparable to the similarly-sized LIGO interferometers. Members of the LSC are currently implementing coherent network analysis methods using maximum likelihood approaches for optimal detection of arbitrary burst signal (see, for example, [40]) and for robust signal consistency tests [41, 42]. Such methods will make the best use of the data collected from the global network of detectors to search for gravitational-wave bursts

Comments:
1. Note a proof that GRW does not exists is just as important from a theoretical and practical standpoint as detection.

2. Time will tell.

Fortunate · #32 (**permalink**) 04-September-2007, 03:04 PM

William,
The last supernova in our local group occurred in 1987. I think the estimated rate of such is one every 30 to 40 years. I think the detectors are at the point that they require an unlikely fortuitous event for a successful detection.
Last I heard, the upgrades on LIGO were scheduled to take seven years (so, in reallity, figure 10-12 years). I don't think LIGO will be operable during the upgrades.

William · #33 (**permalink**) 05-September-2007, 04:55 AM

Fortunate,

Merging black holes, which theoretically should be common in the galaxy, can theoretically generate massive gravitational waves. The following is an excerpt from the January, 2007 paper, GRW review paper. (The paper with 240 authors).

Quote:

A pair of merging black holes emits gravitational waves with very high efficiency;for instance, numerical evolutions of equal-mass systems without spin have found the radiated energy from the merger and subsequent ringdown to be 3.5% or more of the total mass of the system [11].

Quote:

Thus, a binary system of two 10-M⊙ black holes (i.e. Mf ≈ 20M⊙) has fpeak ≈ 750 Hz, and from table 2 we can estimate the hrss sensitivity to be ∼5 × 10−21 Hz−1/2. Using EGW = 0.035Mfc2, we conclude that the reach for such a system is roughly 1.5 Mpc. Similarly, a binary system with Mf = 100M⊙ has fpeak ≈ 150 Hz, a sensitivity of ∼1.4×10−21 Hz−1/2, and a resulting reach of roughly 60 Mpc.

Which would explain why Professor Jim Hough of the Institute for Gravitational Reseach stated "I am optimistic about the chances of a detection over the next eighteen months.". I am sure Professor Hough would not be optimistic if sole the GRW generator was a supernova.

Quote:

Professor Jim Hough of the Institute for Gravitational Research at the University of Glasgow adds "I am optimistic about the chances of a detection over the next eighteen months." When Ladbrokes offered odds of 500-1 against the detection of gravitational waves by 2010, Professor Hough was one of many who were quick to place their bet and the odds fell to 2-1 in days, before the book was closed. The bookmakers could well find themselves paying up in the next 18 months.

Fortunate · #34 (**permalink**) 05-September-2007, 11:08 PM

William,

I am very excited about the fact that LIGO, etc., seem to have improved their chances. Unfortunately, I just learned that LISA has been put on the back burner because of the squeeze on funds (cosmicvariance.com has a thread on this). LIGO and LISA are complementary, since they look at different portions of the wavelength spectrum.

Jerry · #35 (**permalink**) 06-September-2007, 05:54 AM

Quote:

Originally Posted by Fortunate

William,
The last supernova in our local group occurred in 1987. I think the estimated rate of such is one every 30 to 40 years. I think the detectors are at the point that they require an unlikely fortuitous event for a successful detection.
Last I heard, the upgrades on LIGO were scheduled to take seven years (so, in reallity, figure 10-12 years). I don't think LIGO will be operable during the upgrades.

Actually there were a series of upgrades involved in the current science run - Science Run Five (SR5), which should be at or nearing completion. It depends a little bit on who you talk to, or when they wrote it as to whether or not the current level of sensitivity constrains gravitational waves beyond theoretical predictions. Certainly the U.S. Congress was led to believe that the current level of sensitivity should be sufficient to have a high probability of recording a number of events. These are very expensive toys.

It is disappointing that LISA has been delayed, but if there is no hint of success in science Run 5 of LIGO and the collaborating detectors, should the search for gravity waves be batoned off to other wavelengths or abandoned?

A new, upbeat article on gravitational wave detection and the potential impact to science:

http://xxx.lanl.gov/PS_cache/arxiv/p...09.0608v1.pdf1

Caution: Similar glorious predictions were written in the past about today's generation of gravity wave telescopes.

Nereid · #36 (**permalink**) 06-September-2007, 02:05 PM

Quote:

Originally Posted by Jerry

Actually there were a series of upgrades involved in the current science run - Science Run Five (SR5), which should be at or nearing completion. [snip]

SR5 may well be about to end ... but analyses of the data will likely not end for several months, and many key papers - even preprints - based on SR5 won't appear for another year or so. That's an estimate based on the time between the end of previous science runs and when key preprints showed up - check the Einstein@Home page, for papers on SR3, for example, or the LSC publications page.

Fortunate · #37 (**permalink**) 06-September-2007, 07:43 PM

Quote:

Originally Posted by Jerry

...but if there is no hint of success in science Run 5 of LIGO and the collaborating detectors, should the search for gravity waves be batoned off to other wavelengths or abandoned?

To begin to answer your question, we would first need to make a numerical estimate of the probability that this result excludes the existence of gravitational waves in certain wavelengths - we would need to take it out of the realm of optimistic and pessimistic qualitative statements such as "probably," "possibly", "probably not", "very likely," etc.

William · #38 (**permalink**) 06-September-2007, 09:14 PM

The link in Jerry's comment did not work. Attached is a link to "The New Science of Gravitational Waves" by Craig Hogan, and excerpt from the paper.

http://arxiv.org/pdf/0709.0608

Quote:

Laser interferometer detectors measure the relative motions of proof masses. From the ground, detectors such as LIGO, VIRGO, GEO and TAMA probe frequencies of about 30 to 1000 Hz. The strongest sources of waves in that band are expected to be the final stages of coalescences and mergers of stellar-mass binary black holes. In the next decade, these detectors will reach far enough into space to expect an appreciable rate of these rather rare events.

LISA is a proposed space mission designed to measure gravitational radiation over a broad band at low frequencies, from about 0.1 to 100 millihertz, a band where the Universe is richly populated in a wide variety of strong sources of gravitational waves. This rich activity at low frequencies makes sense from an intuitive point of view since after all, astronomical systems are physically large and even with very high velocities most of them do not change very quickly. (In addition, waves from sources at high redshift are slowed further by cosmological time dilation). Indeed, at the low end of its band LISA will be limited by a confusion-limited background of gravitational waves from compact binary stars 2Nevertheless, the existence of ravitational waves is in little doubt as their effects have been measured precisely, if indirectly. The long standing best evidence (Kramer et al. 2006) for gravitational waves is the orbital decay of the Hulse-Taylor binary pulsar, which radiates at frequencies close to the operating band of planned detectors such as LISA. 3 (Farmer & Phinney 2003). (LISA will detect the progenitor populations of the binaries that eventually and rarely flare up as LIGO sources.)

Low frequency signals come from a wide range of different sources: massive black holes merging in galaxies at all distances; massive black holes consuming
smaller compact objects; known binary compact stars and stellar remnants; members of known populations of more distant binaries; and probably other sources, possibly including relics of the extremely early big bang, that are as yet unknown. These strong signals convey detailed information addressing a wide range of science: the history of galaxies and black holes in the Universe; general relativity itself and the behavior of spacetime; precision measurements of the Universe as a whole; the physics of dense matter, stellar remnants and compact binaries; and possibly new physics associated with events in the early Universe, relics predicted in string theory, or even direct detection of quantum gravitational noise in the detector.

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