11
$\begingroup$

With the Big Bang, one would expect the distribution of mass to be somewhat uniform, but we have galaxies that have billions of stars and there are huge distances between galaxies.

Why is there is so much emptiness between galaxies? Like, the nearest galaxy is 25,000 light years from the Sun.

Think of it this way. Our Sun is near the outer end of our galaxy so we have literally a billion stars on one side but like nothing for the next 25,000 light years on the other side (assume we are on the edge of the galaxy for simplicity).

$\endgroup$
  • 2
    $\begingroup$ They're not far apart at all, they're in close groups. Look up at the magellenic clouds, they are enormous because they are so close - they're "right there". $\endgroup$ – Fattie Feb 22 at 13:37
  • $\begingroup$ regarding the distance, you can always ask why that "far", so I guess you are asking: why exactly this distance? $\endgroup$ – Shing Feb 22 at 16:38
  • $\begingroup$ Which galaxy is only 25,000 light years from the Milky Way? (LMC is quoted as 163kly) $\endgroup$ – Michael Feb 22 at 18:17
  • 1
    $\begingroup$ @Michael Ah... the Canis Major Dwarf Galaxy, although its status as a true galaxy is disputed. $\endgroup$ – Michael Feb 22 at 18:19
  • $\begingroup$ It's all relative, I suppose. Compared to their size, galaxies are close together. If you say that the Milky Way is about 100,000 light years across and the Andromeda galaxy is 2,000,000 light years away, that's only 20 diameters. There's even some evidence now that gas gravitationally bound to the Milky Way is interacting with gas "belonging" to Andromeda and star formation may be occurring there. Our galaxies may already be colliding! $\endgroup$ – Jack R. Woods Mar 9 at 0:29
11
$\begingroup$

Most of the universe is pretty empty in terms of the density you're used to in daily life. It's perhaps not that stars and galaxies are far apart, but that they are pretty compact.

This is because baryonic matter (as opposed to dark matter) can lose energy via electromagnetic radiation and hence condense to smaller and denser objects. This is only opposed by angular momentum (which cannot be simply radiated away) forcing disc-like structures such as the Galaxy and proto-stellar and -planetary discs.

One of the obvious answers is gravity, Galaxies are gravitationally bound systems of stars, interstellar gas and dark matter, often hosting a central supermassive black hole. So anything in its hill sphere will fall into the galaxy.

We also know that the universe is expanding which explains the large distance and emptiness because the objects are moving in relation to one another. Neither is moving through space, but space is expanding.

So two galaxies that used to be 1 billion light years apart are now 2 billion light years apart. The expansion of the universe is the formation of new space between the Galaxies.

| cite | improve this answer | |
$\endgroup$
  • $\begingroup$ Is new space also forming inside galaxies or only between them? $\endgroup$ – Leo Feb 22 at 14:36
  • 2
    $\begingroup$ @Leo not only between galaxies, but between everything. Even between your eyes, to put it another way. $\endgroup$ – Sean Mooney Feb 22 at 14:57
  • $\begingroup$ I am not sure it has to do with baryon physics. Dark halos are far apart and their dynamics doesn't involve the physics of baryon (?) $\endgroup$ – chris Feb 22 at 17:00
  • 1
    $\begingroup$ @Leo, To expand on Sean's answer, while the space between your eyes are expanding, the molecular bonds very quickly compensate and stay at the same distance. The expansion is very very small on a human scale, but large at the observable universe scale $\endgroup$ – Fred Stark Feb 22 at 22:46
  • $\begingroup$ Electromagnetic radiation actually can carry angular momentum, albeit only in small amounts - and galaxies rotate far too slowly to make use of what little can be radiated away this way. $\endgroup$ – John Dvorak Feb 23 at 1:28
0
$\begingroup$

According to the current Λ-CDM model of cosmology, vacuum (empty space) has an energy density of 5.4×10−10J/m3, and remains constant as the universe expands. Fitting the Λ-CDM model to precise observations of the cosmic microwave background determines that numerical value. The theoretical reason why the vacuum has this particular value for its energy density remains unknown. This "dark energy", unassociated with any ‘real’ particles, has caused the expansion of the universe, and now causes it to accelerate. It comprises 69.1% of the current total energy density of the universe.

Remember that mass equals energy divided by the square of the universal constant (m = E/c2). The total energy includes this dark energy, plus the energy of all matter (both visible matter and “dark matter”) and radiation.

(Looking at the second part of your question) one could extrapolate a hand-wavy hypothesis that says that since density remains constant as the universe expands, a kind of positive feedback such that ‘empty’ space — the quantum vacuum – creates more and more space and thus more energy and more potential mass as it expands.

This appears to make problems for our conception of mass/energy creation in the 1st law of thermodynamics. Oh well…

| cite | improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.