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I am a part time physics enthusiast and I seldom wonder about the intergalactic space. First, it is my perception that all(almost all) the objects in the universe are organized in the forms of galaxies; or in other words, the universe is a collection of billions and billions of galaxies which are spaced far apart. Does anything actually exist in the empty space between galaxies? If yes, how did it reach there?

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according to wikipedia, it's almost empty there – Mark Eichenlaub Nov 11 '10 at 10:29
please edit your question to obtain a more meaningful title, why not directly the "Does anything actually exist in the empty space between galaxies?" – Tobias Kienzler Nov 11 '10 at 11:22
@Tobias Kienzler - Done! – Sid Nov 11 '10 at 11:58
@SidCool: thanks. That way, should someone else ask a similar question he will already see this under "related questions" – Tobias Kienzler Nov 11 '10 at 12:02
@Mark Eichenlaub: Why don't you post that as answer, maybe quoting a few lines with >? – Tobias Kienzler Nov 11 '10 at 13:24
up vote 10 down vote accepted

I might add some further notes to the actual material things existing in intergalactic space. One might wonder but the notion that there is space is already stating that there is more than nothing.

It implies that there is at least vacuum which is a pretty interesting thing on its own.

Classical harmonic oscillator

Maybe you know that the harmonic oscillator has energy levels

$E_n = \hbar \omega \left( n + \frac{1}{2}\right)$

and an astonishing result is that the lowest energy state is $E_0 = \frac{1}{2}\hbar\omega > 0$.

Quantum electrodynamical oscillator

Coming back to the vacuum, the situation is somewhat comparable. Considering Heisenberg's Principle of Uncertainty in its energy-time form,

$\Delta{t}\cdot\Delta{E} \geq \hbar$

we can see already that a state of a quantum system with definite zero energy for all times cannot exist, even though the expectation value might vanish.
Going more into detail, we see that the operator of the vector potential fullfills the wave equation

$\Delta{A_l} - \frac{1}{c^2}\partial_{tt}A_l = 0$

and a Helmholtz equation if one puts $\partial_{tt}\rightarrow{-\omega^2}$. This equation is usually tackled by separation of variables and after some math we arrive at a Hamilton

$H = \frac{1}{2}\sum_{\lambda}\left({p^2_\lambda+\omega_\lambda^2\lambda{q^2_\lambda}}\right)$

where now $\lambda$ accounts for some mode index. And here comes the magic. This is a description equation for harmonic oscillators! But here we run into a conceptional difficulty. The vacuum energy

$E_{vac} = \frac{1}{2}\sum_\lambda{\hbar\omega_\lambda}$

is infinitely large since there are infinitely many modes of the vacuum. But this is not very physical, so most of the time for calculations you just "leave out" this part.

Implications of a vacuum energy

In the case of different separated domains where you are able to allow a different different number of modes (e.g. via metal plates), this energy will be different for those domains resulting in a force which is the famous Casimir effect.

But vacuum energy has other implications. One hope it that it might some day explain the cosmological constant in terms of a unified field theory.

So, I hope, I could convince you that "empty" might be much more one would expect :)



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+1 you could also mention the connection of $\Lambda$ to dark energy – Tobias Kienzler Dec 14 '10 at 8:12
@Tobias Kienzler: Thank you for the note, of course you are right. There is a lot more to be said about the things mentioned and I indeed had to stop myself at some point. This anwer besides another I posted is also a test if answers to old questions (well, "internet-old", this lasts for one month) will get any feedback or not. – Robert Filter Dec 14 '10 at 8:22
@Robert: the answer to your test: the necromancer badge has been awarded > 8k times at SO – Tobias Kienzler Dec 14 '10 at 8:26
@Tobias: Nice, let's see what happens :) – Robert Filter Dec 14 '10 at 8:34
Thanks...((())) – Sid Dec 14 '10 at 16:43

As others have said, it's almost empty, but not quite, as there are gas particles and so on floating around. As wikipedia states:

Generally free of dust and debris, intergalactic space is very close to a total vacuum. The space between galaxy clusters, called the voids, is probably nearly empty. Some theories put the average density of the Universe as the equivalent of one hydrogen atom per cubic meter. The density of the universe, however, is clearly not uniform; it ranges from relatively high density in galaxies (including very high density in structures within galaxies, such as planets, stars, and black holes) to conditions in vast voids that have much lower density than the universe's average.

And that's only if you consider empty to mean void of matter - there's also electromagnetic waves permeating most (all?) of space. And when you get down to the subatomic level, quantum mechanics ensures that particles are constantly popping into and out of existence as well, even in 'empty' space.

As for how the matter got there, well aside from the normal ways (being shot out of exploding stars and so on), don't forget that before it all started expanding, all of the matter was in the same place anyway, so the particles in intergalactic space haven't necessarily travelled anywhere to get there. They could have simply stayed where they were while particles around them got gravitationally drawn into nearby clumps of matter/galaxies.

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At higher redshifts, we actually see huge clouds of neutral Hydrogen (atomic, not molecular) in the form of absorption lines from distant quasars, called the "Lyman alpha forest"

These clouds may eventually form galaxies and stars, but they are currently just gas, an not particularly low density either.

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probably the only answer that is trying to go over what observations might have to say that is relevant to the question at hand, instead of just cropping sentences together from read-elsewhere speculatia; for you +1 – lurscher Jan 14 '12 at 16:25

A simple picture is the following: stars are organized in galaxies of various types (like elliptical, spiral, ...). These galaxies are organized in groups which are local aggregates of galaxies (our galaxies is in the so-called "local group" of around 40-50 galaxies). The largest structure is a cluster of galaxies: the groups are bound by gravitational interaction in a cluster.

This classification is not really precise but still. A logical conclusion is that the "empty" space in between these cluster is quasi-completely empty, the density of particles being much much lower than in a galaxy (in average) and is uniform: if it was not uniform, the particles would start to gravitationally interact and start to form other structures.

I think that's a valid reasoning for, say, hydrogen gas (atoms). The conclusion as pointed by @Mark Eichenlaub is then that it can be considered empty and that it is often called "the void".

But of course the space cannot be considered empty: the photons (light) traveling from one place to another are as real as other particles.

The next step in the discussion would involve things like dark energy, but that's a different story.

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dark energy is now discussed mentioned in robert's answer – Tobias Kienzler Dec 14 '10 at 8:12

There certainly is gas within galaxy clusters. It tends to have very high temperature, circa a hundred million K, and is detectable by diffuse x-ray emission. Of course we have the elusive dark matter, which is probably more diffusely spread out than luminous matter. So maybe your question should be modified to, "is the space between galaxy clusters a vacuum?". I suspect the answer to that is also no, although the density is clearly very low.

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Try this link for a semipopular view of the overall picture.

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In 1997 the Hubble discovered a large numbers of intergalactic stars. Others have since been discovered. It is now believed that about 1/2 of the stars in the universe may well be rogue stars that are located in intergalactic space. The AVERAGE density of intergalactic space is still very small, however, because of its immense size.

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