If we look back far enough we can see all the origins of the universe, so is it possible, even if not feasible, that we could trace the history of some matter as it moves through space-time? I want to understand how looking at different depths in space and time are correlated in regards to the matter being observed.

For example would it be possible to look deep into a certain part of space and time to find some galaxy that contributed to the matter that makes up the Milky Way today? Then somehow follow it through space-time by looking at different depths and locations in space, and see how it came to be part of the Milky Way?


3 Answers 3


Would it be possible to look deep into a certain part of space and time to find some galaxy that contributed to the matter that makes up the Milky Way today?

No, that's not possible. If we could do that, it'd mean that the matter traveled from there to here faster than its light got here, and matter can't travel faster through space than light does.

All we can do is look at similar galaxies to the Milky Way at earlier times. And because of the expansion of space those galaxies are now even further away from us than they were when they emitted the light that we're seeing now.

Galaxies develop (mostly) in isolation from one another, apart from the occasional merger or collision between neighbouring galaxies. Intergalactic distances are fairly huge, so it takes vast amounts of time for matter to travel from one galaxy to another, and matter is mostly bound by gravity to the galaxy it's in. Galactic escape speeds are pretty high, although the occasional star does get flung out of the galaxy by cataclysmic events like supernova explosions. But even then, such rogue bodies mostly end up in intergalactic space. The odds of them ending up in a different galaxy are pretty slim.

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    $\begingroup$ Milky Way was one example, but my question is more generally, what about other matter? Could we track the formation of a given galaxy through time by looking at different spots and depths in space? And regards to your answer of No for the Milky Way, can we not see the origin of all matter in the CMB, including the matter (or maybe energy) that our Milky Way originated from? $\endgroup$
    – dmoody256
    Mar 10, 2019 at 19:51
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    $\begingroup$ It's the same answer-- you are asking if light from the same object emitted at two different times for that object can be seen by us at the same time. That would require that the object move with its light, i.e., move as fast as the speed of light. Instead, if we watch for ten years, all we can see is the life of that object for a similar timeframe. In fact, objects in the past look like time is going by slowly for them, so you'd end up seeing less than ten years in the life of the object. $\endgroup$
    – Ken G
    Mar 10, 2019 at 20:19
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    $\begingroup$ @dmoody256 I think the confusion arises because the more distant something is, the older the light we receive from it. But we only receive the light from an object once, not as a series of snapshots over time and distance. The CMB, for example, was emitted from matter 13.8b years ago, but we can’t see that same matter at an earlier or later time - that would mean there are multiple copies of the same matter or that the matter moves around the Universe faster than light. The matter in our galaxy doesn’t also exist somewhere else at an earlier time. $\endgroup$ Mar 10, 2019 at 22:36
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    $\begingroup$ @Chappo I was under the assumption that the CMB we see today was all matter/energy shortly after the Bing Bang. So therefore we could see the same matter at that time and now when you look out at the Milky Way, and other depths of space. I'm thinking my assumption about CMB being all encompassing was wrong, but I'm looking into it a bit more. $\endgroup$
    – dmoody256
    Mar 10, 2019 at 23:14
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    $\begingroup$ @dmoody256 that’s why I commented, as the answers don’t address your underlying misconception. The CMB comes from a thin shell of matter about 45b ly away. There’s matter even further away but its light hasn’t reached us yet. There’s matter closer to us, but the light it released at last scattering has already gone past us; we can only see that matter at a later stage of its existence. If we look at Andromeda galaxy, we can’t look further away and see the same galaxy when it was younger: that light has already “gone”. $\endgroup$ Mar 11, 2019 at 3:54

You would have to catch up to the light that carries the information you seek. It's traveled for a few billion years at this point (Earth is ~4.3B). So, you could watch the formation of Earth (Milky Way, whatever), if you could instantly teleport billions of light years away from here.

When we watch distant galaxies and starts, what we're seeing is "old" light. The events that we see occurred many years ago. If we see a galaxy forming, and that galaxy is 10 billion light years away, then that galaxy has already formed. Its configuration at this time is very different from what we see. In fact, some of its stars have already burned out. Similarly, if you lived in (or could instantly teleport to) that galaxy, you would see the Milky Way as it appeared 10B year ago.


To see the Milky Way's formation, you'd need a way to observe the photons that are now ~5 billion light-years away from us.

One way to accomplish this would be to find a mirror in the space far away, and look at our own reflection in it. The gravity of black holes can cause photons to make a full turn, and return to us: https://physics.stackexchange.com/questions/225693/using-a-naked-black-hole-as-a-mirror

However, with our current technology that's still infeasible as the tiny amount of photons from our reflection would be lost among all the others, such as radiation from stars behind the black hole. It would require a huge telescope array to have enough resolution to collect enough and resolve their direction accurately enough. This is only a guess, but it would be something on a scale of billions of telescopes accurately positioned over many light years of distance.

  • $\begingroup$ The idea of using an extra-galactic black hole as a “mirror” to see our own galaxy in the past is ludicrous, and the one reputable answer to that PhysicsSE question makes it clear why. $\endgroup$ Mar 14, 2019 at 11:05

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