Discussion in 'Astronomy, Exobiology, & Cosmology' started by S.A.M., May 9, 2007.
What exactly does it mean when scientists say the universe is expanding?
Log in or Sign up to hide all adverts.
Matter from the original "Big Bang" is still moving outwards.
It means it's contracting.
That space itself is expanding.
The object which we define as the universe is becoming larger. The problem with that is obviously in the definition of the universe. Does the universe include the space which "space" hasn't reached yet? Is there an "outside"?
So how do they define this expanding space? How can tell its the space that's expanding and not just the objects moving further apart?
How would you define the universe? By the space or the objects contained within the space? Or both?
To say the universe is expanding means that, at the level of galaxies and galaxy clusters, over time all the matter in the universe tends to move further apart.
According to the Hubble law, if we look at a distant galaxy, the further away it is, the faster it is moving away from us.
Well I guess I was under the impression that the objects were contained within space. Aren't the two phenomenon distinct from each other?Please Register or Log in to view the hidden image!
Please Register or Log in to view the hidden image!
Does this apply only at the level of a galaxy? Are we moving further away from our sun?
If you're going to posit that the expansion has something to do with individual objects (like galaxies) and not space itself, then you need to ask why the observed expansion is so uniform. For example, why is it that the speed v of every distant galaxy is related to its distance from us x by v = Hx, where H is a constant that is the same for all galaxies? Another interesting observation is that seemingly, with all galaxies apparently moving away from us, we are at the centre of the expansion. Does that mean Earth is the centre of the universe after all? (Hint: physicists say no.)
How is expanding space(time) modelled? It is modelled using the best theory we have for dealing with this kind of thing: Einstein's General Theory of Relativity. The theory allows us to postulate a universal "structure" for space and time. We then go out and collect evidence to see if the model matches the facts. Current models have a number of "adjustable" parameters, which leave us free to do some adjustment when we get new data, but many many models are absolutely ruled out by the currently available evidence. Of course, physicists would like to do better than this, and show that one particular model with no free parameters is the only possible description of our universe's spacetime. They're working on that.
We are not moving further from the Sun. Theoretically, the expansion of space applies to everything, but on "small" scales, such as the scale of an individual galaxy (and certainly our solar system), the relatively small influence of the universal expansion is overwhelmed by local gravitational influences.
Can we be certain that what we are measuring is what is happening? Is the model extrapolated at such distances?
There's no distance extrapolation per se. Also, I should say that the validity of the big bang model does not depend solely on stuff at the far ends of the visible universe. The model makes a number of predictions about how the universe ought to look - e.g. it predicts the rough ratio of hydrogen to helium, it predicts the existence of the observed microwave background radiation and the observed fluctuations in that, and so on.
As for certainty, few things are ever 100% certain. All we can say is that, based on current physics, astrophysicists are pretty sure they have the universe's large-scale behaviour fairly well nailed.
This is not to say that there aren't some major puzzles which still need sorting. The glaring ones at the moment are the puzzles of dark matter and dark energy.
Maybe I misunderstood, don't we need to measure the distance of the distant galaxy from us? I was just wondering how exact that science is,
It's reasonably exact.
Distances are measured in astronomy by building up a "ladder" of distance scales, starting from things that are close to us and gradually working outwards to similar objects we can see with telescopes.
For example, we can fairly directly determine the distance from Earth to the Sun. Then, by looking at the orbits of the other planets we can determine how big the solar system is. We can use certain methods (e.g. parallax) to determine the distances to the nearest stars. We can classify stars into certain types according to their spectral colours, and we know what the absolute luminosity of each kind of star should be. Using that, and the apparent luminosities of distant stars of the same type (the type can be determined in various ways), we can determine the distances to many of the stars in our galaxy, and even the distances to some stars in other galaxies.
Looking at the motion of nearby galaxies, we discover the Hubble law. Since we can measure the recession speeds of stars and distance galaxies very accurately (from spectral details), we then can determine the distances of even very distant galaxies. This process is complicated by a number of factors (e.g. gas and dust between us and the distant objects), but we know how to correct for that.
So, in summary, we have a fairly good handle on universal distances. Uncertainties are well quantified.
James, what is your opinion on what is going to happen to the universe? Is it going to turn into a dark and dead metal junkyard?
Isn't an explosion a fairly uniform expansion? And that is not the space itself expanding, right? Besides, the Hubble constant measure some form of force that is perceived with a redshift. That doesn't mean the redshift is an evidence for the universe expansion.
Yes. And because of this I question whether the universe is actually expanding or not. Everything that expands, expands from a central point. Take for instance a ballon. When it expands, it has a centre from which it expands. Without that centre there is no way to perceive an expansion. The universe has no centre. So how come the universe is expanding?
Another issue that, in my opinion, disproves the expansionary theory is that we actually observe a blueshift with a few galaxies. For instance, the local group is actually getting closer together. Andromeda will be colliding with us in a few billion years and the Magellean Clouds will be colliding with us pretty soon. Now, you can argue that that is due to the strong gravitational force of those galaxies' proximity compared to galaxies far away. However, the Virgo cluster is one example of an extra-galactic object that is actually moving closer to us (blueshift). So how would you explain that blueshift?
I mean.. my question is... what if the redshift doesn't mean the universe is expanding? There's one way that could be happening- rotation. Picture this... do you see the tires of a car? Well, if you pick any point on it (let's say X) while it is rotating, you will see that the points that are away from X and on the direction of the rotation (let's say Y points) may appear to be moving away from Y if you compare with points Z, which are "behind" point X, given the direction of the rotation. So in this model, from the perspective of X, most points would appear to be moving away (because they are ahead in the rotation), while a few points would appear to be moving closer (because they are right behind). Given the age of the universe, I would assume the correct interpretation to be a past redshift (galaxies that have a redshift are seen from a past perspective) while blueshifts would be a look into the future.
Actually, it might be the other way around. Maybe local groups get closer together and eventually become quasars...
In which case we have a bright future ahead.. LOL!! Please Register or Log in to view the hidden image!
Separate names with a comma.