Why is the universe so big?

Question

The Universe is approximately 13.7 billion years old. But yet it is 80 billion light years across. Isn't this a contradiction?

Answer 1

This question implicitly refers to the visible universe, but we should state that explicitly, as otherwise the question doesn't make any sense.

It may seem like we shouldn't be able to see more than 13.7 billion light-years (13.7 giga-light-years, or glyrs) away, but that reasoning omits the expansion of spacetime according to General Relativity. A photon emitted from somewhere near the beginning of the Universe would have traveled nearly 13.7 glyrs if you had measured each light-year just as the photon crossed it, but since those light-years that you measured have expanded since the photon passed through, that distance now adds up to about 80 glyrs.

Answer 2

The universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space.

No one knows if the universe is infinitely large, or even if ours is the only universe there is.

Although our view of the universe is limited, our imaginations are not. Astronomers have indirect evidence that the universe of galaxies extends far beyond the region we can see. But no one knows if the whole universe is infinitely large - large beyond limit.

According to the leading theories, other parts of the universe may look very different from our own - and may even have different laws of nature. We may never be able to find out for sure. But it is possible that clues to the answer lie in plain view, just waiting to be discovered!

I should note also the "80 billion light years across" it doesn't count as a contradiction. I don't know what your reference is but I believe that this concerns the region that we can see yet of this Universe.

Answer 3

Yes this does sound like it might be a contradiction but in fact it isn't. It's because the universe has been expanding rapidly in every direction since the big bang and our observations are limited by the speed of light.

For example if we observe a distant quasar that appears to be 10 billion light years away, the light from the quasar is 10 billion years old (which is why quasars are known for being some of the most ancient phenomena in the universe). In the time it took for that light to reach us the universe has been expanding. In fact the expansion has been accelerating all that while so the distance between us and that quasar is at this present time considerably larger than 10 billion light years.

If we had a means to observe distant objects as they appear right at this instance, not only would we have a type of time machine but we might observe the universe to be 80 billion light years across, although I cannot vouch for the accuracy of the 80 billion light year figure given on the wikipedia page.

Astronomers and physicists also struggle with the question of "what is it that the universe expanding into". Is it expanding into empty space and if so is there anything that lies out there beyond our universe at some vast distance? If so, the universe may be infinite.

Or does the universe fold back in on itself on some higher dimensional plain i.e. if we had a hypothetical means to travel faster that the rate of the expansion of the universe and travelled in a straight line, would we eventually end up back in the same spot? In this scenario the universe would have a theoretical boundary at any one time.