If GWs propagate at near the speed of light, how did Earth (and its constituent elements) arrive at its current location billions of years before the GW arrived here (or light from distant stars for that matter)?
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1$\begingroup$ The Earth follows the movement of the Sun due to its gravitation. And the Sun moves inside the galaxy according to its initial momentum, when it (the Sun) was created. But I feel, that it's not the answer you're looking for. $\endgroup$– GeorgeCommented Sep 27, 2017 at 17:59
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4$\begingroup$ The Earth didn't come from the place that GW are coming from. $\endgroup$– A. C. A. C.Commented Sep 27, 2017 at 18:07
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$\begingroup$ @A.C.A.C. - Didn’t we all come from the same singularity (Big Bang) though? $\endgroup$– iMerchantCommented Sep 27, 2017 at 18:17
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2$\begingroup$ @iMerchant That would assume that both the Earth and GWs came into existence at the same time during creation of the universe with the Big Bang. The Earth is only 4.5ish billion years old. $\endgroup$– zephyrCommented Sep 27, 2017 at 19:09
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$\begingroup$ @iMerchant: The far-away objects haven’t moved away from us, though; instead, the intervening space has expanded. Related: astronomy.stackexchange.com/questions/14336/… $\endgroup$– chirluCommented Sep 27, 2017 at 19:46
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2 Answers
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Let's look at GW150914, the first directly detected gravitational wave event. The source was a binary black hole, with a calculated luminosity distance of $410^{+160}_{-180}\text{ Mpc}$ - roughly 1.3 billion light-years, give or take. This means that the signal was produced about 1.3 billion years ago. The system itself is much older, of course; the progenitor stars formed about 10 billion years before the merger.
Earth didn't come onto the scene until roughly 4.5 billion years ago. At that point, the binary black holes had yet to merge together; the system was producing gravitational waves, but nothing as strong as the ones detected by LIGO. The gravitational waves we detected were produced 3 billion years after the Earth was formed - and it took them 1.3 billion more to reach Earth!
The point is, the signal LIGO detected was created after Earth was formed, and created far away. They did not start out at the same time Earth did.
answered Sep 27, 2017 at 19:55
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$\begingroup$ +1 for the cool looking formula that makes zero sense to me. But I totally understand what you are saying overall. $\endgroup$ Commented Sep 27, 2017 at 20:33
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$\begingroup$ @iMerchant: It just means that the distance could be between 230 Mpc (410–180) and 570 Mpc (410+160), but that 410 Mpc is the best estimate. $\endgroup$– chirluCommented Sep 27, 2017 at 21:08
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1$\begingroup$ Non astronomer here. Lol. Mpc = miles per century? Lol. Don’t know mpc. Sorry for my ignorance. Just trying to learn new things like everyone else. $\endgroup$ Commented Sep 27, 2017 at 21:47
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$\begingroup$ @iMerchant: Mpc = megaparsecs. A parsec is roughly equal to 3.26 light-years, so a megaparsec is 3.26 million light-years. 410 Mpc is therefore 1.34 billion light-years. $\endgroup$ Commented Sep 27, 2017 at 22:20
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1$\begingroup$ Jim421616 - And 13 parsecs = Kessel Run. Got it. $\endgroup$ Commented Sep 27, 2017 at 22:26
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Earth didn't get here before GW's. GW's have been around since the early universe. We have just recently developed the technology to detect the large gravitational waves associated with very massive disruptions in the fabric of space-time.
We also know that our sun is not a 1st generation star, so much of the matter used to create our sun and planets came from a much earlier super-nova explosion.
