LIGO Detects Gravitational Waves for Third Time
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Re: LIGO Detects Gravitational Waves for Third Time
#2Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?
Re: LIGO Detects Gravitational Waves for Third Time
#3I was at a talk by Janna Levin, astrophysicist and author of a book Black Hole Blues that describes LIGO. (E.g., https://www.nytimes.com/2016/04/18/books/review-black-hole-b...)
She gave a neat analogy between GWs, as sensed by LIGO, and an electric guitar. In the sense that a distant pluck on the string is transmitted as a wave down the string to the pickup, which senses a little wiggle in the string and amplifies it. I thought it was a poetic analogy that gives a second meaning to the word "instrument" in this context.
Re: LIGO Detects Gravitational Waves for Third Time
#4Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?
While there are many compact binary systems in the universe continuously emitting gravitational waves for a very long time, LIGO is only able to detect the most violent waves emitted by the final coalescence and merger. So, on the astrophysical side, there is some joint probability given by how common these systems are, and how likely they are to merge in a given time frame. (Space based observatories like LISA would be able to see the long-lived inspiral waves though.)
On the instrumental side, we've only just reached the sensitivity levels to make any detections in the first place, so it's not surprising that we're not getting a huge number of events (otherwise the previous generation of detectors would've seen something). In addition, each individual observatory has its own "antenna pattern", making us less less sensitive to certain sky locations. This will improve as VIRGO, KAGRA, and LIGO-India come online in the future.
Re: LIGO Detects Gravitational Waves for Third Time
#5Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?
Events big enough to be detected are quite rare, but the phenomenon you are asking about is more a financial reality than a "convenient coincidence" as you put it. There is a pretty steep curve connecting sensitivity and cost, so when the team that built LIGO was designing it, they used the best available models of colliding black hole event rates to estimate the sensitivity required to deliver a conclusive result in a reasonable amount of time.
If you're getting 10 events/second with a device like this, you probably overpaid for sensitivity and if you're getting 1 event per century you're probably not going to be able to maintain the operating expenses to still be running when the detectable event occurs (and, as critically, none of the people involved will be able to get the data they need in the time they need it to get their PhD's, assistant profesorships, or tenured positions, so you can't get the labor force you need for your experiment to work on it, which is really what sets the acceptable duration of most experiments in practice).
It looks like the original estimates were pretty good, so events are coming in at about the rate the experimenters hoped they would see them.
Re: LIGO Detects Gravitational Waves for Third Time
#6The paper describing the event is available to the public at https://dcc.ligo.org/LIGO-P170104/public
The instrument data of this event is also available to the public at https://losc.ligo.org/events/GW170104/
Re: LIGO Detects Gravitational Waves for Third Time
#7Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?
I'm not a physicist/astronomer so someone could give you a more detailed explanation. From what I understand, LIGO has been designed to detect collisions of mid-sized black holes, and it looks like those aren't that common.
There's also a more paper on this issue which you can find here: https://arxiv.org/pdf/1704.04628.pdf
Re: LIGO Detects Gravitational Waves for Third Time
#8Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?
Events big enough to be detected are quite rare, but the phenomenon you are asking about is more a financial reality than a "convenient coincidence" as you put it. There is a pretty steep curve connecting sensitivity and cost, so when the team that built LIGO was designing it, they used the best available models of colliding black hole event rates to estimate the sensitivity required to deliver a conclusive result in…
Hmm...are the rates of black holes per volume well-constrained at all? I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs? The truth or falsity of this would seem to have a big effect on rates.
Re: LIGO Detects Gravitational Waves for Third Time
#9Somewhat naive questions, as I know very little about astronomy. Do black holes "move?" How is it that they could merge if they're stationary, unless they're pulling each other in I guess? If black holes are indeed pulling in everything, does that mean the whole universe would eventually be one giant black hole?
Re: LIGO Detects Gravitational Waves for Third Time
#10> These are collisions that produce more power than is radiated as light by all the stars and galaxies in the universe at any given time.
Astounding, especially given that these are happening at regular intervals in our "neighborhood".