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The Ligo website says "Detecting the relic gravitational waves from the Big Bang will allow us to see farther back into the history of the Universe than ever before."

I find this puzzling. How can a Gravitational Wave originating at about the same time and place as the particles that make up their detector (ie in the Big bang) be detected? To use a simple analogy - an explosion in an electronics warehouse causes a flash of light and pieces of electronics gear to be scattered. These pieces are then assembled to make a camera which is able to photograph the explosion. I am confused!

Answers suitable for a high school physics class greatly appreciated.

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    $\begingroup$ You could use exactly the same argument for the cosmic microwave background. The big-bang is not an explosion into some other space. $\endgroup$
    – ProfRob
    Mar 11, 2016 at 10:14
  • $\begingroup$ Rob, I don't think the OP knows about the CMB. And if you don't question whether or not the Big Bang was an explosion in space, but just think it is, I think it's difficult to search for previous answers. I think that's why there are so many related questions on this topic. $\endgroup$
    – pela
    Mar 11, 2016 at 10:45
  • $\begingroup$ But maybe instead of answering the question, I should've found a previous answer and explained why it also answered the OP's question. $\endgroup$
    – pela
    Mar 11, 2016 at 10:47

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Firstly, Big Bang didn't happen at a point in space, away from which we are traveling. Big Bang was the creation of space. This space has been expanding ever since, so that the distances between everything increases, but Big Bang happened right where you are, where the Andromeda galaxy is, where GN-z11 is, and so on.

The Universe has evolved ever since. If we want to know how galaxies look 13.8 billion years (Gyr) after Big Bang, we can just look around in our neighborhood. If we look too far away, we don't see 13.8 Gyr old galaxies, because the light has taken some time to reach us. Thus, if we want to see galaxies that are 12.8 Gyr old, we simply look 1 billion lightyears away; if we want to se 10 Gyr old galaxies, we look (roughly) 3.8 billion lightyears away, and so on.$^\dagger$ In this way, we look back in time.

If we look away far enough, we would in principle be able to look 13.8 Gyr back in time. However, we face a problem in that until the Universe was 380,000 years old, it was opaque to light (why this is so is another story). It wasn't opaque to gravitational waves, however. And since lots of GWs are thought to have originated during the epoch called inflation, which were responsible for the expansion of the Universe, and which took place a fraction of a second after the creation of space, we say that GWs offer the possibility of looking all the way back to Big Bang.

$^\dagger$This is somewhat imprecise, since galaxies weren't created instantly after Big Bang, and since all galaxies weren't created at the same time. But for the sake of the arument, let's pretend they were.

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  • $\begingroup$ This is so confusing (to me at least). Don't u mean the BB happened where we (or the precursors to the Milky Way/solar system/etc) were 14B years ago? $\endgroup$
    – iMerchant
    Mar 11, 2016 at 12:55
  • $\begingroup$ @iMerchant: Yes, BB happened everywhere. All of space was created at BB. At that time all of space was immensely hot, and all of space started to expand. Space doesn't expand into something (like an explosion), it just expands. Some time after BB, all of space had cooled enough to start forming galaxies, everywhere. None of these galaxies are in the center, or at the edge, of an explosion. Or you could say all of them are. Just like no human can claim to be at the center of Earth's surface. Or you could say all of them can. $\endgroup$
    – pela
    Mar 11, 2016 at 13:38
  • $\begingroup$ I thought everything, according to the BB, quickly expanded from an infinitely tiny singularity and was super hot, then cooled, like you said. (Sounds "explosive" to me) Are you saying all the primordial ingredients of the Milky Way, for instance, were the size of less than a pin prick? In other words, if the MW of today had existed during the time of the BB, it would have fit on the head of a head of a pin I can hold in my hand? Or are you saying "The Big Bang" terminology is somewhat misleading or not exactly the best description/title of what the theory describes? $\endgroup$
    – iMerchant
    Mar 12, 2016 at 4:59
  • $\begingroup$ But then again, I have an accounting background, not astrophysics. I'm not extremely well-versed in all this, just always been naturally curious about stuff. $\endgroup$
    – iMerchant
    Mar 12, 2016 at 5:07
  • $\begingroup$ @iMerchant: The observable Universe — i.e. the part we can see (and in which we are centered because we can see equally far in all directions) — is finite, and at BB it was a pin prick. The rest of the Universe may be finite or infinite. If it's finite, then all of the Universe was a pin prick at BB, containing an even smaller pin prick which is "our" bubble, which in turn held the seed for the MW. Not a pin prick expanding in an empty space, but a pin prick that was all that is, and then expanded. [cont'd below] $\endgroup$
    – pela
    Mar 12, 2016 at 11:59

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