If I am right, we can see only those stars that lie within our cosmological horizon, and there may or may not be any stars beyond that. Given last 150 years of using telescopes, and since then our cosmological horizon must have increased by about 150 light years, have we seen any new galaxies, stars? If given enough time, say 500 years, new stars are found, can we be sure they have become visible against not being formed, say, 150 years after big bang?

  1. No. Space expansion doesn't slow down, unlike the expansion of cosmic horizon. That means the cosmic horizon actually swallows galaxies as they travel away from us due to space expansion, reducing their number instead of increasing it. Nevertheless, we still haven't gathered all cosmic radiation from Big Bang that occurred within our current event horizon, so it's still a good while before it begins restricting distance of observable universe for us - currently the speed of light (+space expansion rate) vs age of the universe limits it to a considerably smaller radius.

  2. Yes, we discovered more stars and galaxies, but that is due to improvement of technology, which is still quite far (roughly a third of the way) from observing stars near the Hubble Sphere as the universe formed (ones whose light would be observable today; as opposed to these at the space horizon, whose light will reach us eventually, in infinitely distant future.)

Currently we're still quite a bit away from leaning against the cosmological limits of observability. Currently our limit of observability is the budget for building better devices.


There are a couple of different horizons you should care about. The first is the cosmological horizon, which is the furthest you could possibly see given that photons travel at a finite speed and the universe is not infinitely old. Since nothing travels faster than the speed of light, this is quite literally the furthest we could ever hope to see - everything outside is causally disconnected from us (though this horizon does increase with time, it asymptotically reaches some ultimate distance due to the accelerated expansion of space-time).

Practically speaking, the horizon which we really care about is the cosmic microwave background. This is the point at which the universe became cool enough such the atoms could remain neutral. Why is this the important horizon? Before this, photons simply could not travel very far before interacting with charged particles. Before the surface of last scattering the universe was essentially opaque (it would be like looking through a cloud). Afterwards, however, it was transparent. Everything from the surface of last scattering and later (redshift of $z\sim 1100$ and lower) is said to be part of the observable universe.

Though the universe may still be infinite in size, we will never be able to see these objects (if they exist) if they are passed this horizon. The other thing to remember is that the further you look, the further back in time you're seeing. Galaxies did not always exist. It took time to form larger overdensities like stars, galaxies, and clusters from small perturbations in density during the early universe. After the surface of last scatter, there was a period of time in which the universe was very dark and potentially very boring from an astronomer's point of view. This is the time after atoms became largely neutral yet before the first stars and galaxies "turned on".

  • $\begingroup$ This answer is incorrect. Recession at the speed of light does not mark any horizon. The answer fails to mention the two actual cosmological horizons, the particle horizon (our past light cone at current time, very roughly marked by the CMB) and the event horizon (our past light cone at infinite time) which are two very different concepts. The answer also seems to mix up the event horizon and the particle horizon. Nothing passes the particle horizon in the outward direction. $\endgroup$ – Thriveth Dec 23 '14 at 0:41
  • $\begingroup$ Also, calling the dark ages "boring" from an astronomer's point of view is not serious. Many astronomers are very interested in this topic. $\endgroup$ – Thriveth Dec 23 '14 at 0:44
  • $\begingroup$ Firstly, I say "boring" because there weren't objects to look at in this stage of the history of the universe. People do study the swiss cheese universe at high redshift using radio surveys, but you are wrongly interpreting what I mean here, as stars and proto-galaxies have to have formed first - which is not what I am discussing here. $\endgroup$ – astromax Dec 23 '14 at 15:32
  • $\begingroup$ You also incorrectly interpret my first point. I am not saying the speed of light is a hard cut-off in what we can observe, however, I am saying that since the universe expands (in a non-trivial way), and the speed of light is finite, there is a fundamental limit to how far into the past we can see. In other words, it DOES have to do with the speed of light. Your definition of the particle horizon is a rather poor answer, since you fail to make it understandable in any way. $\endgroup$ – astromax Dec 23 '14 at 15:37
  • $\begingroup$ Lastly, welcome to the astronomy stack exchange :) This is probably not deserving of a separate comment, but feel free to inject strict definitions of the cosmological and particle horizons (for those of us who worry about them), or clarify the language in my above answer. $\endgroup$ – astromax Dec 23 '14 at 15:57

Over the past 150 years, telescope technology has improved immensely, so we can now see much further than in the past. Hubble's 'Deep Field' images would not have been possible 30 years ago, and are among the first I know of that show objects near the cosmological horizon.
Also, while the cosmological horizon increases over time, objects near the horizon have a huge redshift (speed) away from us. I don't have the data to prove it, but I suspect objects near the cosmological horizon would disappear past the horizon over time.

  • $\begingroup$ Things would actually get redder and redder, so you'd actually have to change the wavelength of your observations. They wouldn't technically disappear though. $\endgroup$ – astromax Nov 2 '13 at 21:54
  • $\begingroup$ The cosmological horizon recedes at 1 lightyear per year. It was my understanding that faraway objects have redshifts that indicate much higher speeds than that, so they'd eventually overtake the cosmological horizon. $\endgroup$ – Hobbes Nov 4 '13 at 10:26
  • $\begingroup$ The expansion of space is the other thing you have to think about. Yes, all things being equal our horizon increases by 1 ly/year, but the expansion of space (which if it's the same everywhere as it is here), is locally 70 km/s/Mpc, and when you look at distant objects they're moving much faster than that number. Each 'cell' essentially of size 1 Mpc has its own local version of this number, and therefore they add. Locally nothing can travel faster than c, but that doesn't mean globally the coordinates of space must obey that rule. This is why the horizon will essentially asymptote. $\endgroup$ – astromax Nov 4 '13 at 12:40
  • $\begingroup$ What I'm saying is that things you currently see would become redder and redder as the expansion accelerates. At the moment the expansion is not larger than c (globally or locally). $\endgroup$ – astromax Nov 4 '13 at 12:41
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    $\begingroup$ @astromax In fact, anything with a redshift $\gtrsim 1.4$ is receding from us faster than light. $\endgroup$ – Thriveth Dec 23 '14 at 0:49

As far as observation goes, distance equates to age, so as our technology improves we effectively see farther into the past. The actual increase in distance visible to us is much much greater than 150 light years.

We can actually detect the effects of the big bang itself, in the microwave background radiation of the universe, but sars and galaxies formed much later than 150 years after themig bang (try millions of years :-)

  • $\begingroup$ Actually, the CMB formed around 300.000 years after the Big Bang. $\endgroup$ – Thriveth Dec 23 '14 at 0:44

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