Have Gravitational Waves Been Detected?(universetoday.com)
universetoday.com
Have Gravitational Waves Been Detected?
http://www.universetoday.com/110353/rumors-flying-nearly-as-fast-as-their-subject-have-gravitational-waves-been-detected
11 comments
I understand that LIGO can detect something, but how do they know where it came from? How can they register a disturbance in the spectra to a a source? Do they really quickly point a ccd at the sky and hope they captured what happened?
CMB telescopes use microwave antennas (often feedhorns) to direct the electromagnetic radiation onto some kind of detector. In the case of BICEP, the detector is a superconducting bolometer, which senses small temperature changes caused by CMB photons depositing themselves onto a resistive element.
To get data, a CMB telescope is scanned across the sky for several months (or years), and the raw time signals are converted into a map of the microwave sky.
LIGO is an entirely different category of scientific instrument; it is looking for the direct influence of present-day gravitational waves on the motion of terrestrial masses.
To get data, a CMB telescope is scanned across the sky for several months (or years), and the raw time signals are converted into a map of the microwave sky.
LIGO is an entirely different category of scientific instrument; it is looking for the direct influence of present-day gravitational waves on the motion of terrestrial masses.
I wonder if they're still going for LISA (> ligo). I guess LIGO wasn't sensitive enough? http://lisa.nasa.gov/
The first gravitational wave detectors weren't sensitive enough; there is a second generation (including a more advanced version of LIGO) that is supposed to come online in the next few years.
In parallel, people are also going forward with development for LISA. It had to be scaled back a bit because NASA decided to pull out, but it's still going.
In parallel, people are also going forward with development for LISA. It had to be scaled back a bit because NASA decided to pull out, but it's still going.
Just as a reason for doing this in parallel. Space based detectors (such as LISA) are sensitive to a different frequency range than ground based detectors such as LIGO and thus to different of astrophysical phenomena [1]
[1] http://www.ast.cam.ac.uk/~rhc26/sources/
[1] http://www.ast.cam.ac.uk/~rhc26/sources/
Yeah, I think this is the interesting part, it wasn't detected by the "gravity wave detectors" but by something else.
I think that they can point the source because the effects they detected means it could have only come from one place.
I think that they can point the source because the effects they detected means it could have only come from one place.
wouldn't arrival of the same event at four or more points (at different times) determine the original sphere center of the wavefront?
How did we get ahead of the waves? Or did they wrap around a curved universe and come back to us?
IANAC [1], but here's my Wikipedia-level 2 cents:
(Warning: leaky metaphors ahead!)
We stayed ahead of the primordial gravitational waves [2] for so long because the expanding universe went through an "inflation" period: in a very short time - immediately after the Big Bang - spacetime expanded at a rate that was many orders of magnitude too big for phenomena like light or gravitational waves to catch up to.
It leveled off quickly to a more gentle rate of expansion [3], but in the meantime the universe has gotten so big that even its current age of 13.7 billion years isn't enough for us to be able to observe every object. Many objects are so far from us that it will take much more than 13.7 billion years for their photons to reach us. And no matter how distant the objects that will 'come into view' in the future, the light that appears the most distant to us will always be the cosmic microwave background radiation.
So here we find ourselves, Earthly observers, sitting in the center of a sphere with an apparent radius of 13.7 billion lightyears - called the observable universe - at the edge of which we can see the immediate aftermath of the Big Bang. No one knows how much bigger the total universe is w.r.t. to the observable universe. It's hard to find out because we have no causal relation to anything outside the observable universe. A safe bet IMHO (given past experiences in the history of cosmology) is this: the entire universe is way, way, way bigger than the observable universe.
[1] I Am Not A Cosmologist
[2] and also of the cosmic microwave background radiation
[3] sub light speed, at least for distances within the observable universe
(Warning: leaky metaphors ahead!)
We stayed ahead of the primordial gravitational waves [2] for so long because the expanding universe went through an "inflation" period: in a very short time - immediately after the Big Bang - spacetime expanded at a rate that was many orders of magnitude too big for phenomena like light or gravitational waves to catch up to.
It leveled off quickly to a more gentle rate of expansion [3], but in the meantime the universe has gotten so big that even its current age of 13.7 billion years isn't enough for us to be able to observe every object. Many objects are so far from us that it will take much more than 13.7 billion years for their photons to reach us. And no matter how distant the objects that will 'come into view' in the future, the light that appears the most distant to us will always be the cosmic microwave background radiation.
So here we find ourselves, Earthly observers, sitting in the center of a sphere with an apparent radius of 13.7 billion lightyears - called the observable universe - at the edge of which we can see the immediate aftermath of the Big Bang. No one knows how much bigger the total universe is w.r.t. to the observable universe. It's hard to find out because we have no causal relation to anything outside the observable universe. A safe bet IMHO (given past experiences in the history of cosmology) is this: the entire universe is way, way, way bigger than the observable universe.
[1] I Am Not A Cosmologist
[2] and also of the cosmic microwave background radiation
[3] sub light speed, at least for distances within the observable universe
Some corrections:
* When I wrote "we stayed ahead of the [waves] for so long" I didn't mean that gravitational waves from the Big Bang & its immediate aftermath haven't been hitting us in the past. On the contrary: gravitational waves and background radiation from this event have always kept reaching us as our observable universe expanded. I was only referring to these particular waves that are hitting us now.
* I insinuated we will always see the CMB radiation but that might be too strong a statement. When (if ever!) our observable universe has fully expanded to encompass the entire universe, we will no longer see radiation originating from the Big Bang.
* I think we should stay skeptical about the existence of gravitational waves as they remained undetected for so long and also this time there's the possibility that someone might have jumped the gun.
* When I wrote "we stayed ahead of the [waves] for so long" I didn't mean that gravitational waves from the Big Bang & its immediate aftermath haven't been hitting us in the past. On the contrary: gravitational waves and background radiation from this event have always kept reaching us as our observable universe expanded. I was only referring to these particular waves that are hitting us now.
* I insinuated we will always see the CMB radiation but that might be too strong a statement. When (if ever!) our observable universe has fully expanded to encompass the entire universe, we will no longer see radiation originating from the Big Bang.
* I think we should stay skeptical about the existence of gravitational waves as they remained undetected for so long and also this time there's the possibility that someone might have jumped the gun.
In my layman understanding current prevailing theory is that acceleration of expansion will reduce observable universe, and as consequence there is basically no question of "size beyond observable universe" - there is no possible interaction with it and as such it doesn't exist.
http://en.wikipedia.org/wiki/Accelerating_universe
http://en.wikipedia.org/wiki/Accelerating_universe
> In my layman understanding current prevailing theory is that acceleration of expansion will reduce observable universe, and as consequence there is basically no question of "size beyond observable universe" ...
Yes, but that was true before the Dark Energy discovery. It's predicted to become much worse in the far distant future because of the acceleration caused by Dark Energy, and eventually there will be comparatively small island universes -- galactic clusters -- that will remain gravitationally bound far into the future, after the remainder of the universe has receded from view. This is because galactic clusters are below the threshold for being affected (i.e. torn apart) by Dark Energy.
> there is no possible interaction with it and as such it doesn't exist.
Yes to the first, no to the second. At present, we cannot see beyond a certain time horizon, but cosmological curvature measurements take this invisible mass-energy into account even though it's not directly observable. Our present conjecture that the universe is infinite in size (based on curvature measurements) obviously implies a universe most of which isn't visible, but all of which affects the measurements.
Yes, but that was true before the Dark Energy discovery. It's predicted to become much worse in the far distant future because of the acceleration caused by Dark Energy, and eventually there will be comparatively small island universes -- galactic clusters -- that will remain gravitationally bound far into the future, after the remainder of the universe has receded from view. This is because galactic clusters are below the threshold for being affected (i.e. torn apart) by Dark Energy.
> there is no possible interaction with it and as such it doesn't exist.
Yes to the first, no to the second. At present, we cannot see beyond a certain time horizon, but cosmological curvature measurements take this invisible mass-energy into account even though it's not directly observable. Our present conjecture that the universe is infinite in size (based on curvature measurements) obviously implies a universe most of which isn't visible, but all of which affects the measurements.
Most physicists[1] would dispute the conclusion that "as such it doesn't exist." They would simply say that there can be no interaction with anything beyond the horizon---that stuff out there is certainly... out there.
[1] myself included
[1] myself included
Just a slight correction: The distance from Earth to the edge of the observable universe is 46 Gly, not 13.7 Gly, because of the metric expansion of space.
True. The light has been traveling for "only" 13.7 billion years, but meanwhile the space between origin & destination kept expanding. That's why I wrote about an apparent radius, but on further thought I no longer believe that the travel time will reflect in an apparent distance. It's the amount of redshift that will result in a calculated proper distance.
Any echo of the big bang would be coming from all directions and going in all directions. It did not leave from here and come back, it left from some other point in the universe and is just reaching us.
/not-a-scientist
/not-a-scientist
That's how the current estimation of the universe age was made (only it was about the background space temperature of about 4 Kelvin). This time, by measuring magnetic waves they seek to find a (even slight) difference in expanding directions.
/not-a-scientist-either
/not-a-scientist-either
Think of the universe like a low resolution digital image. When you zoom in, every part of the larger image is still part of the smaller image. And so the universe expanded in a way that every part of the universe would be an original starting point from before the expansion. Thus these waves would be coming from all directions rather than just being "ahead" or "behind" us.
[deleted]
Sean Carroll's explanation of how this detection of gravitational waves could set constraints on inflationary theories is helpful:
http://www.preposterousuniverse.com/blog/2014/03/16/gravitat...
http://www.preposterousuniverse.com/blog/2014/03/16/gravitat...
What has always puzzled me about this is why anyone would think that a local change in a gravitational field would not propagate as a wave. What the hell else could it do?
I do believe there are differential equations involved that make the field at one point dependent on the field at another point after a period of time and that's all that's required.
I do believe there are differential equations involved that make the field at one point dependent on the field at another point after a period of time and that's all that's required.
It's not that simple, due to the gauge invariance of the theory (not all mathematical degrees of freedom correspond to physical ones) and the approximations involved in deriving wave solutions. The argument was finally settled by Feynman's "sticky beads": http://en.wikipedia.org/wiki/Sticky_bead_argument
>why anyone would think that a local change in a gravitational field would not propagate as a wave. What the hell else could it do?
propagate immediately. Wave is a propagation with finite speed (and the expectation is for gravitational waves to have the same "c" ). There wouldn't be "wave" in immediate propagation, i.e. with infinite speed.
propagate immediately. Wave is a propagation with finite speed (and the expectation is for gravitational waves to have the same "c" ). There wouldn't be "wave" in immediate propagation, i.e. with infinite speed.
Are their no other testable aspects of GR that would show gravity changes to have finite propagation speed?
The assumption for the past 100 years has been that gravity propagates as a wave -- as mentioned in the article, it's the last untested prediction of Einstein's general relativity. But conclusive evidence has not been discovered.
Untested, untested.. Directly observed is a better definition, the quasar observations are quite convincing.
http://downforeveryoneorjustme.com/www.cfa.harvard.edu
> It's not just you! http://www.cfa.harvard.edu looks down from here.
> It's not just you! http://www.cfa.harvard.edu looks down from here.
Not sure why this isn't top of HN. Isn't this huge?
Stream Monday 17th, 15:55 UTC -
http://www.cfa.harvard.edu/news/news_conferences.html
Stream Monday 17th, 15:55 UTC -
http://www.cfa.harvard.edu/news/news_conferences.html
> Not sure why this isn't top of HN.
Gravitational waves beating a story about someone who Hula hooped for a rapt audience of geeks? Get serious. :)
> Isn't this huge?
It could well be huge. Gravitational waves are the last confirmation of general relativity that so far has no direct observational evidence.
I say "direct" because pulsars (compact, very massive collapsed stars that emit radio pulses) have been observed to slow their pulse rates over time consistent with the idea that they're radiating away some of their kinetic energy in the form of gravitational waves. But this is indirect evidence at best, and there are other possible explanations for the decline in pulse rates.
Gravitational waves beating a story about someone who Hula hooped for a rapt audience of geeks? Get serious. :)
> Isn't this huge?
It could well be huge. Gravitational waves are the last confirmation of general relativity that so far has no direct observational evidence.
I say "direct" because pulsars (compact, very massive collapsed stars that emit radio pulses) have been observed to slow their pulse rates over time consistent with the idea that they're radiating away some of their kinetic energy in the form of gravitational waves. But this is indirect evidence at best, and there are other possible explanations for the decline in pulse rates.
The headline doesn't help, that's for sure. The fact is that gravitational waves have been detected already, though indirectly, and a Nobel prize was awarded for the observations more than 20 years ago. That's not why the BICEP2 results are a big deal.
The reason this news is important is two fold. One, the researchers were able to observe the effect of "b-mode" gravitational waves during the earliest moments of the big bang (within the first 10^-34 seconds) which reveal important details about the specifics of the big bang. They are indirectly observing an event which is older than all matter in the Universe, older than the cosmic microwave background, older than the cosmic neutrino background, older than dark matter even (which has a high likelihood of originating from the very early Universe when the 4 fundamental forces were still unified). Two, their observations are the first direct test for the theory of cosmic inflation which seems to have produced a definitive result and the result is significant evidence for inflation.
That, in turn, translates some theories into realities. For example, we tend to hedge our bets when it comes to estimating the size of the Universe, because only so much of it is visible to us. In a Universe where inflation holds the non-visible Universe must be unimaginably huge compared to the visible portion of it by many orders of magnitude. Also, most inflationary theories effectively produce multiverses (this is hard to explain, basically it's like a foam of different regions of the universe which are effectively universes unto themselves due to speed of light / expansion rate constraints).
The reason this news is important is two fold. One, the researchers were able to observe the effect of "b-mode" gravitational waves during the earliest moments of the big bang (within the first 10^-34 seconds) which reveal important details about the specifics of the big bang. They are indirectly observing an event which is older than all matter in the Universe, older than the cosmic microwave background, older than the cosmic neutrino background, older than dark matter even (which has a high likelihood of originating from the very early Universe when the 4 fundamental forces were still unified). Two, their observations are the first direct test for the theory of cosmic inflation which seems to have produced a definitive result and the result is significant evidence for inflation.
That, in turn, translates some theories into realities. For example, we tend to hedge our bets when it comes to estimating the size of the Universe, because only so much of it is visible to us. In a Universe where inflation holds the non-visible Universe must be unimaginably huge compared to the visible portion of it by many orders of magnitude. Also, most inflationary theories effectively produce multiverses (this is hard to explain, basically it's like a foam of different regions of the universe which are effectively universes unto themselves due to speed of light / expansion rate constraints).
I'm looking forward to this as much as I did NASA's 2010 secretive massive announcement of extraterrestrial life.
s/extraterrestrial life/arsenic-based life on earth
s/arsenic-based life on earth/something maybe
s/extraterrestrial life/arsenic-based life on earth
s/arsenic-based life on earth/something maybe
There's a good amount of preparation at Stanford astrophysics underway for discussion of the results from the BICEP2 collaboration tomorrow.
If you see a headline that asks a yes-no question, the answer is almost always, "No."
If you don't know anything about the subject then deciding the answer to a yes/no question without any data or logic is a useless exercise.
In this case the answer is almost certainly "yes", not least because gravitational waves have already been detected anyway, though as with the current experiment indirectly.
In this case the answer is almost certainly "yes", not least because gravitational waves have already been detected anyway, though as with the current experiment indirectly.
Damn! The one time I quote Betteridge's Law (thanks, hemmer), and it turns out the answer is "yes".
So far this is just hype, speculation, and an "aspiring science journalist" (says so in the byline) trying to make a name for herself.
If she's right, big scoop and plaudits to her.
If she's wrong, nobody cares. She covered her ass with a question mark.
Smart woman. She'll go far, I'm sure.
If she's right, big scoop and plaudits to her.
If she's wrong, nobody cares. She covered her ass with a question mark.
Smart woman. She'll go far, I'm sure.
The significance is that this would be an experimental observation of cosmic inflation. How it is observed (gravitational wave) is less important.