emmm....how do we know that the universe

[j000z11]

I've heard from peoples that the universe is constantly expanding at the speed of light and I was wondering how do we actually know that? I just had a little thought on the back of my head today and I think I want to share it with people in UM. I was thinking that what if they universe is not acutally expanding? Instead, its strinking? Example: A basketball ball filled with airs(represents unoccupied space), sands(stars, planets, or matters), and a small hole in it(represents black holes). Like when a red supergiant star explodes into supernova, and after it turns into a black hole which is when it creates a hole in the ball. Like a basketball, when there is a hole in the ball, it starts shrinking and sucks out all the airs and dirts inside the ball. and the rest I haven't thought of yet.... so please give me your opinion... I don't have much knowledge of Physics, Astronomy, or other sciences. But I love them and would like to learn them so if I have any mistakes please correct it and it would be very much appreciated.

[Lilly]

Pretty much everything indicates that the universe is expanding. Hubble's Law is based on observational evidence that the redshift in light coming from other galaxies is proportional to their distance (ie, the other galaxies are moving away from Earth).

Here's a nifty link.

[Trinitrotoluene]

Hi j000z11,

That is infact a very good question. First of all though, I'd like to clear something up, for you and others about the expansion of the universe. The important thing to remember, is that the expansion of space is metric. It is not an expansion in the way that most people understand it. It is usually incorrectly assumed that it's like the expansion in an explosion, for example, but this is not the case. It is not the objects that are moving, it is the space inbetween them. The best way to visualise this, is imagine raisins in a piece of bread put into the oven. When the bread expands, the raisins move away from each other, all though it is important to note that the expansion is isotropic, as in there is no central point to this expansion. This is where Hubble's law comes into the equation. Hubble's law states that the redshift in light coming from distant galaxies is proportional to their distance. By looking at this redshift, we can see that the galaxies that are further away are receding faster than the ones that are closer. Remember that there is no central point to this expansion, so this supports the statement that the universe is expanding.

Just as a side note, I thought I might explain what redshift it. It is actually as simple as it sounds, it's a shifting of the light, towards the red end of the spectrum. This occurs when an object is moving away from us. The best way to explain this, is in terms of sound. Imagine an ambulance going past you. As it is approaching it has a higher frequency sound than when it has passed you, it has a lower frequency. It is a very similar scenario with light instead of sound. When we look at the light from galaxies, we see that the light has been shifted towards the red end of the spectrum, and thus we can conclude that it is moving away from us.

Any questions feel free to ask

Edited July 6, 2007 by Trinitrotoluene

[j000z11]

That was very helpful, thanks Lilly and Trinitrotolueno

[Primeval]

Impossible question, your mind cant get around it.

[Captain Kolak]

Oh, but remember. Gravity also affects light. So some dark matter might be affecting light inbetween galaxies slowing light down. And the further a galaxy is, the more light is slowed. But in some areas the affect is opposite and light is speeded up.

Lol... never new i could make somethin so crazy up in 30 seconds. But hey, only a few thousand years ago people people believed in only four elements that made up the whole universe.

[keithisco]

Hi j000z11,

That is infact a very good question. First of all though, I'd like to clear something up, for you and others about the expansion of the universe. The important thing to remember, is that the expansion of space is metric. It is not an expansion in the way that most people understand it. It is usually incorrectly assumed that it's like the expansion in an explosion, for example, but this is not the case. It is not the objects that are moving, it is the space inbetween them. The best way to visualise this, is imagine raisins in a piece of bread put into the oven. When the bread expands, the raisins move away from each other, all though it is important to note that the expansion is isotropic, as in there is no central point to this expansion. This is where Hubble's law comes into the equation. Hubble's law states that the redshift in light coming from distant galaxies is proportional to their distance. By looking at this redshift, we can see that the galaxies that are further away are receding faster than the ones that are closer. Remember that there is no central point to this expansion, so this supports the statement that the universe is expanding.

Just as a side note, I thought I might explain what redshift it. It is actually as simple as it sounds, it's a shifting of the light, towards the red end of the spectrum. This occurs when an object is moving away from us. The best way to explain this, is in terms of sound. Imagine an ambulance going past you. As it is approaching it has a higher frequency sound than when it has passed you, it has a lower frequency. It is a very similar scenario with light instead of sound. When we look at the light from galaxies, we see that the light has been shifted towards the red end of the spectrum, and thus we can conclude that it is moving away from us.

Any questions feel free to ask

Might be worth adding that the expansion of space occurs in the macro universe, which means that expansion occurs in inter-galactic space (the gaps between galaxies). Within galaxies themselves, local Galaxy groups, and within solar systems the effect of expansion is negated by the effect called gravity.

Your analogy with the raisins in the bread is excellent in visualising this.

There is one bug in this... Andromeda galaxy and the Milky way (our galaxy) are actually moving towards each other and the light from Andromeda is blue shifted. The relative velocity is in the order of 500,000 Kmh. This is due to the fact that they are what is termed a "Bound-Pair" - bound by gravity. Not time for us to worry though, it will be 3 billion years befor the two Galaxies begin to merge!