PDA

View Full Version : Expansion

cryobot11
2010-Feb-10, 06:16 AM
I've heard that space is expanding at approx 70.4 km/s. And that nothing with mass can go faster than C. You can appear to be going faster than the speed of light due to your velocity being added to this 70.4 km/s expansion. My question is what is causing space to expand at this rate and more importantly how was it discovered?

astromark
2010-Feb-10, 06:42 AM
... how was it discovered?...--- Both by direct and indirect observation.

...what is causing space to expand at this rate?... ---I have not the slightest notion or idea.

...You are correct in your assumption that matter can not reach velocity c. If the void of space is expanding at a ever faster rate then yes, it might and does delegate some objects as red shifted to oblivion...

Jens
2010-Feb-10, 07:00 AM
I've heard that space is expanding at approx 70.4 km/s.

That's not quite right. It's about 70 or 74 km/s/Mpc. In other words, if you look out one Mpc, you see something receding at 70 km/s, and an object two Mpc away will be receding at 140 km/s. I won't respond to your question about the speed of light. But the answer to the first is that if you look at faraway galaxies, they are redshifted in a way that gives this.

DrRocket
2010-Feb-10, 07:04 AM
I've heard that space is expanding at approx 70.4 km/s. And that nothing with mass can go faster than C. You can appear to be going faster than the speed of light due to your velocity being added to this 70.4 km/s expansion. My question is what is causing space to expand at this rate and more importantly how was it discovered?

Observational evidence from type 1a supernovas, objects of known luminosity and redshift provded the first evidence for an accelerating expanison of the universe.

http://arxiv.org/PS_cache/astro-ph/pdf/9805/9805201v1.pdf

http://en.wikipedia.org/wiki/Type_Ia_supernova

http://www.lbl.gov/Science-Articles/Archive/oldest-1a-supernova.html

Nobody knows what is causeing the accelerated expansion. It has been given a name, "dark energy" and is it modeled as a positive cosmological constant in the Einstein field equations that are used in general relativity.
There are some ideas as to the fundamental cause, but none that are consistent with known physics.

Relativity does not permit objects to travel faster that light through space, but places no limit on the recession velocity that can come with expansion of space itself.

Your expansion rate of 70.4km/s is a bit misleading, since it indicates a simple rate and the rate is in fact linearly dependent on distance. I suspect that you are referring to the Hubble constant which is currently estimated at 72 +/8 km/s/Mpc (http://en.wikipedia.org/wiki/Hubble's_law)

cryobot11
2010-Feb-11, 06:19 AM
Thanks for all your responses....awesome support. Sorry for the wrong info about the Hubble constant. After further research I figured that out...but again, thanks. I love that there are so many people willing and happy to share knowledge. Makes this hobby that much more interesting and keeps it fun!!!!

George
2010-Feb-12, 03:38 PM
My question is what is causing space to expand at this rate and more importantly how was it discovered? The earliest hints of expansion came when Vesto Slipher was asked by Lowell to take spectrums of what were then thought to be nebulae. In 1912, Slipher was the first to discover redshifts in most of these nebulae -- later these nebulae were found to be galaxies -- and redshifts strongly implied very fast recessional speeds. [This was no small accomplishment and it meant cold all-nighters to capture enough photons to get any sort of reliable image.]

Later, Edwin Hubble put an older telescope back into service, which had been donated by a doctor from England, I think. He, with the help of Milton Humason (8th grade education and former observatory janitor), was able to demonstrate that the more distant a galaxy, the faster it was receeding from us, though a few close galaxies like Andromeda were coming toward us.

The redshift in the spectral lines of a spectrum reveal how fast the galaxies are receeding from us, but Hubble needed some way to pin down just how far each of these galaxies are away from us in order to determine what would become known as the Hubble Constant (ie expansion rate).

The key for distance was found in the Cepheid variables, which are stars that vary in brightness by a set amount based on their period for this brightness -- a discovery by Henreitta Leavitt in 1908. Once Hubble found some Cepheids in these galaxies, he was able to produce the first sound evidence for the expansion. [Cepheids, however, were later found to come in different "flavors" and Hubble was thrown off by this in the original expansion estimate.]

The Type 1a supernova studies revealed that the expansion is now accelerating, as has been mentioned by others here.

nokton
2010-Feb-12, 04:07 PM
... how was it discovered?...--- Both by direct and indirect observation.

...what is causing space to expand at this rate?... ---I have not the slightest notion or idea.

...You are correct in your assumption that matter can not reach velocity c. If the void of space is expanding at a ever faster rate then yes, it might and does delegate some objects as red shifted to oblivion...

Hi Astro, try to conceive gravity as a negative quantity, then what happens inside a
bubble, when the bubble expands. The conditions within the bubble determine it's
surface tension, it's surface tension holds it together, for a while...
In one manner of speaking, the universe is a bubble of spacetime.
Nokton