# The Size Of The Universe?

## Recommended Posts

I was recently given a book as a present. Wonders Of The Universe, By Brian Cox & Andrew Cohen. The book of the TV series.

The introduction starts....

At 13.7 billion years old and 45 billion light years across... the universe....

If we accept that nothing (we know about) can travel faster than light. Also accept the universe expanded from a point. Then the universe cannot be larger than it's age, 13.7 billion light years. Or perhaps you double this for an edge to edge measure? Either way it is less than 45 billion light years.

Can someone knowledgeable please enlighten me on where my thinking is wrong?

##### Share on other sites

Don't have the time to give a full explanation now, but "nothing can travel faster than light" is probably the most commonly quoted and misleading statement in astronomy.

Nothing can travel through space faster than light, the universe itself can travel as fast as it likes

It's to do with things such as co-moving distance, I gave wiki a quick scan and seems pretty accurate http://en.wikipedia.org/wiki/Observable_universe#Size

##### Share on other sites

Good point above.

Another way to express it; the light travel time is 13.7 billion years. While that light has been in flight, though, the universe has been expanding so the starting point has receeded by the time the observer observes the light.

X throws a ball to Y. Y catches it after 3 seconds ball flight time. The ball was flying at 10 metres per second. So X is 30 metres away? If no one has moved, yes. But if X and Y have been legging it in opposite directions during the flight of the ball, no! They are more than 30 metres apart at the time of the catch.

There are two other 'issues' in your post. The universe, in the BB model, did not exactly 'expand from a point.' It was a point and all that there was expanded. The difference might seem trivial but I don't think it is. I think that conceptually the difference matters. Ignore the difference and you start thinking of explosions or, worse, explosions into something and this makes 'getting it' even harder. Bad enough as it is!!!

Also there are no edges. Forget edges, they are not there in the BB model and will only confuse you. Think of horizons instead.

Olly

##### Share on other sites

I googled size of the universe and came up with this,

"The age of the universe is about 13.75 billion years, but due to the expansion of space we are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological proper distance, which is equal to the comoving distance at the present time) than a static 13.75 billion light-years distance. The diameter of the observable universe is estimated to be about 28 billion parsecs (93 billion light-years), putting the edge of the observable universe at about 46–47 billion light-years away."

Whatever that means...

##### Share on other sites

Photons can be seen about 300,000 years after the Big Bang.since the density of universe is lower while the free path of Photons is higher.

##### Share on other sites

The Wiki article is very good.

This has of course brought up the difference that (probably) exists between the size of the observable universe (assuming very good observation kit) and the size of the universe in total.

##### Share on other sites

Books to look into.

Why does e=m(cxc) by Brian Cox and Jeff Forshaw

Universe by Freedman and Kaufmann

##### Share on other sites

There is a lot of stuff here. Has to be the most universal mindbender. Beyond me.

Speed of light is near 300,000 km/s in a vacuum. (It is slower in any other medium). The universe and any matter, mass cannot move faster than the speed of light. Note Einstein e=m(cxc), theory of relativity.

Age of the universe is accepted at <14by. Hubble Law accounts for recession.

Earth is not geocentric in the universe as it is not, so Copernicus said, for the Solar System. There is no centre.

The universe looks the same from any galaxy remote or not.

There is no edge. As Olly stated - horizon.

Universe size?? Conjecture. An hypothesis requires support in terms of observation. At this time, the universe is as above <14by until proven otherwise. But we can be sure it is bigger.

First Light Out was when the "big bang" cooled to, I think if I remember correctly, 3000K when sub atomic particles, quarks, slowed to enable protons and electrons to combine to form hydrogen atoms. The dark universe lit when this occurred - photons could be emitted and absorbed - 300,000.

A universal mindbender.

#### Archived

This topic is now archived and is closed to further replies.