Have Gravitational Waves Been Detected?

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.

wouldn't arrival of the same event at four or more points (at different times) determine the original sphere center of the wavefront?

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.

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

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

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.

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.

>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.

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

> 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.

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).

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.