Originally posted by jiggerj
Is anyone claiming that this is the edge of our universe and that there is nothing beyond it?
The CMB is not coming at us from any source. It pervades the whole universe.
Originally posted by ImaFungi
Would there be a gigantic "empty area" in the middle of the universe where the singularity occurred?
Further elaboration (of both answers) below.
In its very early stages, the universe was mostly hydrogen, and it was dark. It was not until a million or so years later (numbers vary), when the
first stars began to shine, that light dawned on the universe. The first light in the
During these 'Dark Ages' the universe was expanding amazingly fast – much
faster than the speed of light. This we call
. The reason space can expand faster than light is that space is not matter; only
material substances are speed-limited by relativity.
If the inflation hypothesis is correct, then the most distant parts of the universe are still
travelling away from us at this speed, carrying
away with them the stars, galaxies, etc. they contain. The light from these objects will never reach us; they have always been invisible to us, and
forever will be.
But later in the history of the cosmos, the acceleration slowed drastically. This explains why the nearer (hence more recent) an object is, the more
slowly it seems to be moving away from us.
The parts of the universe we can see are those moving away at speeds less than that of light. We call this volume of space the 'observable
universe'. The oldest and farthest objects in it are 13.7 billion years old, but their distance from us is much greater – about 46.5 billion
light-years. This is due to the expansion of space described above. If space had not been expanding, the farthest objects we saw would still be only
13.7bn LY away.
The expansion of space also 'stretches' the light from the most distant visible objects so much that they appear to us as long-wave radio signals.
Ancient Radio Waves Hold Key to Universe's First Light
phenomenon is known as cosmological redshift
and is different from the Doppler
redshift with which most people are familiar.
But the visible universe isn't the whole universe. Inflation implies that the cosmos almost certainly extends well beyond that 46.5bn LY visibility
boundary. How much farther is unknown, since we have no way of accurately measuring or estimating the rate or duration of the inflation epoch. Indeed,
the universe may, for all we know, be infinite (though most scientists think this unlikely).
What the invisible portion of the universe contains, how big it is and so on are things we shall probably never know. All scientific estimates of the
size, mass, energy, etc. of the universe are concerned only with the 46.5bn LY sphere we call the 'observable universe'.
Size of the universe
The expansion of space (both now and during the early inflation phase) was not expansion away from a central point. Space is expanding everywhere
throughout its fabric, so every point in the universe is the centre. As far as we humans are concerned, Earth is the centre of the universe (no,
really!) That is why the CMB seems to be coming at us from every part of the sky – and why it would look just the same to an observer in a distant
galaxy, even (probably) one beyond the fringes of the observable universe.