# Expanding universe

1. Apr 8, 2004

### Amstelerdam

Hello all,

I am from Amsterdam, the Netherlands. I am an super-amature on this topic but I have a question.

Is it not possible that the universe is expanding because the amount of matter is growing?

See it like a bucket filled with water(universe), throw a rock(matter) in the bucket and the water level will rise(expand).

I hope you people understand what i try to ask, my english is not so very good.

2. Apr 8, 2004

### Phobos

Staff Emeritus
Welcome to Physics Forums, Amstelerdam!

Due to the finite speed of light, the further away we look into the universe, the further into the past we see (it takes light longer to reach our eyes from more distant objects). If your idea was correct, then we would expect to see less mass in the past (in other words, fewer & smaller galaxies in the distance) than we do nearby. But we don't see that, as far as I know. I suppose someone could do the math to figure out how much mass would have to be appearing in order to cause the expansion we measure.

Also, if the amount of mass in the universe were less in the past, then the amount of gravity in the universe would have been less and, therefore, the "evolution" of the universe would have been different than what we see.

There's also the problem of "where does this mass come from" and "what form does it appear as" and "is it uniformly distributed"? Fresh influxes of hydrogen may cause us to see more "1st generation" stars than we actually see (older stars contain other elements that were formed from earlier generations of stars). Nearby galaxies could be brighter/more active than we currently see.

I'm sure there are many more problems that other forum members will mention soon.

3. Apr 8, 2004

### billy_boy_999

it's easier to say that the universe is not expanding and matter is not expanding either - space-time is expanding independent of matter and matter is just along for the ride...ruimte-tijd is groeien en materie is drijven ermee, dus het groeien verder terzijde...

4. Apr 10, 2004

### Newton

why does this idea of more matter explain anything?

amsterledan, why were you lead to this idea?

5. Apr 10, 2004

### Amstelerdam

Well in my amaturisitic mind it seemed logical. If you put something into nothing, nothing must make room for the something... eeeh you understand?

It will not explain anything, but perhaps why the universe is expanding.

And what Phobos explained earlier seems right, I dont have enough knowledge on this subject to say anyhting.

Why? If our solar system was only made up of the sun and earth would our gravity been less or more than 1g? And how can you be sure that the "evolution" of the universe would have been different.

I also would like to know if there are indeed less galaxys when we look "into" the past.... If the answer is yes, than maybe just maybe I am right :)

Maybe someone knows ?

Thanks..

6. Apr 11, 2004

### yogi

Perhaps the size of the universe and its mass are interdependent. There are some very interesting cosmic relationships - for example the numerical density of a critical mass universe is approximately equal to the inverse of the Hubble radius... and the estimated mass of the universe is approximately numerically equal to the surface area of the Hubble Sphere. It is not necessary to consider that energy increase takes the form of more particles - it can come about by stretching space (if space is under tension as in a false vacuum undergoing continuing inflation). In this model of the universe, expansion produces energy by increasing the spatial stress - - and since this stress acts upon the surface area of the Hubble sphere it is easy to see how or why the total energy of the universe is related to its surface area.

7. Apr 11, 2004

### Mike2

When you say "size of the universe", how can that have any meaning unless there is a boundary to the universe. What is the boundary of the universe? If there is no boundary, then how can you talk of size?

8. Apr 11, 2004

### yogi

Mike 2 -- All that we have knowledge of is what is contained within the volume of our Hubble sphere - if there is something beyond, it is probably never going to be observed - this does not imply an infinite universe - if you scale down the dimensions to a two sphere universe e.g. the surface of an inflating balloon, a two dimensional being will consider the entire surface as his Hubble universe - it is finite yet unbounded - the same is true of our three dimensional universe - there is not necessarily anything beyond the Hubble sphere - it is perhaps one sphere of a hypersphere - there may be a forth dimension in which it is embedded - but there doesn't have to be - the topology of a three dimensional universe that is curved is described by a metric equation - just as we can describe all points on the surface of an inflating balloon with two numbers. In summary, we can always make calculations based upon the size of what is observable (knowable within the tenants of SR and GR )