# Does anything exist in the intergalactic space?

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?

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.

## Quantum Mechanical 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 :)

Sincerely

Robert

• +1 you could also mention the connection of $\Lambda$ to dark energy Dec 14 '10 at 8:12
• @Robert --nice post +1. Sidney Coleman was a great guy with a wicked sense of humor. One of his famous papers is titled "Why is There Nothing Rather Than Something". Lubos posted a great lecture of his on his blog --"Quantum Mechanics in Your Face". Feb 18 '11 at 15:25
• Why was this accepted as the correct answer?! It answers a completely different question, that of quantum mechanical void. The question was about intergalactic void - again, completely different cases. Apr 11 at 13:20

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.

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.

• 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 Jan 14 '12 at 16:25

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.

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.

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

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.

Some of the comments and answers suggesting that the space between galaxies is near-empty appear to be somewhat misleading (although, note that some of the answers - like the answers by @Grant Crofton or @Cedric H. - seem to address the space between galaxy clusters rather than between galaxies). Other answers elude to the info below, but here is some clarification about the intergalactic, intracluster space, which from a large-scale viewpoint is very far from empty.

Even without considering dark matter, the majority of matter in galaxy clusters it not in the galaxies, but rather between them in the intracluster medium (ICM), which is a plasma consisting mostly of hydrogen and helium ions (observable in the X-ray portion of the electromagnetic spectrum). In galaxy clusters, around 85% of the mass is in the ICM rather than galaxies / stars. (Then, dark matter makes up even more of the cluster's mass.)

Of course, since galaxy clusters have diameters spanning multiple megaparsecs, the density of the intracluster medium is very low compared to a planet or star. However, the main idea here is that most of the (even baryonic) matter in the universe is actually in the "empty space" between galaxies - so it doesn't make much sense to view that space as "empty" at all.