Cosmology probes mysteries humans have wondered about since we first looked at the stars. Some answers are emerging.
Is space infinite?
Current models (supported by experimental evidence) assume the Universe is infinitely big and has been for the 13.7 billion years since it sprang into existence.
But language throws much confusion into the picture. We can only see part of the Universe ¯ only within a sphere, cantered at Earth with a radius the distance light travels in the 13.7 billion-year age of the Universe. The rest of the Universe is invisible to us.
What readers asked:
"So, I would say that stating the Universe is definitely infinite may be slightly overstating our claim, since we don't really know that it is infinite beyond our observable universe, but we have no strong reason to believe it is not infinite," astrophysicist Tamara Davis e-mails.
Also, the expanding nature of our Universe complicates understanding. A photon travelling for 13.7 billion years traverses more than 13.7 billion light years in distance, because the Universe is expanding. The photon moves through a space about three times larger (46 billion light years).
So, we should speak of two Universes ¯ the Entire Universe and the Visible Universe but often sloppily call them both by the same term: the Universe. That's the rub. So, when cosmologists speak of a "grapefruit" size early-time Universe, they mislead us, big time.
What cosmologists mean is the part of the Universe we now see (present Visible Universe) used to be grapefruit sized, astrophysicists Charley Lineweaver and Tamara Davis write in Scientific American. But the Entire Universe then (as now) could be infinite. "Shrink an infinite space by an arbitrary amount, and it is still infinite."
We must, however, exclude what happened at the time of the 'Big Bang', since we know nothing about that instant. 'Big Bang' time is when time began and our theories have validity only up to Planck time (10^43 seconds) after 'Big Bang' time.
Is there an actual centre of the Universe?
No there isn't a centre of the Entire Universe. Just a centre (at Earth) of our Visible Universe.
What's at the end of space?
The Universe doesn't end, as counter-intuitive as this seems. Once again, language confuses concepts. We speak of a 'Big Bang' but don't mean a 'bang' like an explosion, which has a centre and a shock wave that moves spherically out into air from the explosion.
Instead, the 'Big Bang' happened everywhere in the Universe at once, with no centre. Shortly after the 'Big Bang', density and pressure of the Entire Universe were the same everywhere. So, pressure difference could not possibly create an explosion. The 'Big Bang' wasn't a bang at all.
About 13.7 billion years ago, the Entire Universe increased by 10^30 (a million trillion trillion) times, in less than a second. We call this remarkable phenomenon, the 'Big Bang.' The Universe simply expanded ¯ not into anything because, even at the beginning, the Universe was probably infinite. (We can't say for sure the Universe was infinite at the exact beginning since our theories are valid only up to Planck time after the Big Bang). Thus, the Universe probably has no centre.
Sometimes analogies help understanding strange concepts like 'no end' and 'no edge.' Imagine we're a couple of ants living on a big red balloon. We think as two-dimensional creatures. We can't go up or down and move only on the balloon's surface. I, unfortunately, am stuck in a spot of honey and cannot move. But we wonder how big our world is. Where does it end? I'm fearful you'll fall off the edge but you insist on exploring the balloon. No matter which direction you set off, though, you always come back to me but can find no end to our world.
To complicate our life, but not enough to defeat us, a kid is blowing up the balloon as you wander. So, each circuit you make around our world takes longer than the one before (even though you crawl at the same ant speed) but you always come back to me and never encounter an edge. We conclude our world is expanding, distances are increasing, but our world has no end.