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G.D.
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Forgive my ignorance?.. If we can see 13.8-ish billion light years away how can the universe be the same age? Matter cannot travel at the speed of light, so how are we as far away (in light years) as the universe is old?
What we see are the light rays that have been traveling for about 13.8 billion years.G.D. said:Forgive my ignorance?.. If we can see 13.8-ish billion light years away how can the universe be the same age? Matter cannot travel at the speed of light, so how are we as far away (in light years) as the universe is old?
How can we determine that those things are 45 billion ly? The red shift?Doppler?marcus said:We routinely see stuff that is now between 45 and 46 billion LY away. But we see it as it was around year 380,000 of the expansion, not as it is now.
Relativity allows distances to grow at several times the speed of light---that is not subject to the same rules as ordinary motion, stuff moving thru its surrounding space.
Yes, matter can't travel faster than the speed of light, it's the space that is expanded.G.D. said:Hello and thank you both. So let me see if I've got this. The universe is 13.8 ish billion yrs old, but in that amount of time the universe has expanded to the point where our furthest viewable objects are 46 billionish light years away. Matter can't travel faster than the speed of light, but the space inbetween galaxies can expand faster. So if something is 46 billionish light years away that doesn't mean it's that old.
Ahh, that's the answer. Sorry, just haven't read the incoming threads. Is it the combination of doppler shift factor and Hubble law?marcus said:Hi G.D. would you like to know how to calculate that 46 billion LY for yourself? It is a simple integral from early time up to present, so if you have taken (even very beginning level would do) calculus, you might not be put off by the integral and enjoy using it to get the distance to the farthest matter we can see.
there is a website called numberempire.com that does integrals for you online. it's easy to use and free. You just go there, paste or type in the function you want to integrate over some range----and put the limits (start and finish of the desired range) of integration in and press calculate.
Basically the integral is telling you how far a flash of light can travel in 13.8 billion years when it is helped by expansion. You add up all the little cdt steps the light takes multiplied by how much each step gets expanded between the time it takes the step and the present. dt is a bit of time, cdt is the original length of the step, and then there is the expansion factor S(t) and the integral adds all these little S(t)dt steps up. We use units where c=1 so we don't have to include the speed of light explicitly.
If you don't want to bother with the integral yourself, it's fine, there's also an online calculator called "Lightcone" that does all that stuff for you. I keep the link in my PF signature to have it handy.
We can measure the light emitted by the object then. From this combined with the knowledge of how the universe expands we can calculate the distance then, about 46 million lys, and the distance now, 46 billion lys; assuming that the universe expanded by a factor of 1000.Stephanus said:Yes, matter can't travel faster than the speed of light, it's the space that is expanded.
But how we measure that the object is 46 billion lys?
Special relativity doesn't work in an expanding universe, as special relativity assumes flat space-time, and the expansion is curvature in space-time.Stephanus said:How can we determine that those things are 45 billion ly? The red shift?Doppler?
And from what I learned in SR Forum.
Supposed V = 0.99c, so red shift is ##k = \sqrt{\frac{1+V}{1-V}} = 14## Even if its V is 0.99...99c still if we multiply it by the age of the universe it can't be farther than 13.5 billion ly.
So how can we know that this particular thing is farther than 13 billions ly?
Thanks for the answer.
Chalnoth said:The redshift that we see for objects far away in the universe has nothing to do with the Doppler shift
Good point! Things do have their individual motions in the local space surrounding them and that does contribute a doppler bit on top of the main distance expansion redshift.Gaz said:Well that can't be true I think its safe to assume that the galaxies would have been moving around independently somewhat too ?
But I guess no one can really know how much ?
I was a little bit short in that explanation, you're right. Especially within very massive clusters, the local motions can be pretty large, but not large compared to the cosmological redshift for far-away galaxies. The largest are, if I recall correctly, about 3,000 km/s, which translates to a redshift of ##z = 0.01##. Most galaxies are going to have much smaller local motions (our own motion relative to the CMB is about 600km/s). Obviously for galaxies at a redshift of 1-2 or higher, this isn't going to be an issue (for the most part). Most cosmologists just ignore the local motions of galaxies entirely, and that approximation works extremely well.Gaz said:Well that can't be true I think its safe to assume that the galaxies would have been moving around independently somewhat to ?
But I guess no one can really know how much ?
The current estimated age of the observable Universe is approximately 13.8 billion years old. This age is based on measurements of the cosmic microwave background radiation and the expansion rate of the Universe.
Scientists use a variety of techniques, including measuring the redshift of galaxies and the cosmic microwave background radiation, to calculate the age of the observable Universe. These measurements are then used to estimate the expansion rate of the Universe and its age.
No, we cannot observe the Universe at its current age. Due to the finite speed of light, the observable Universe only allows us to see objects as they were in the past. The farther an object is from us, the longer it takes for its light to reach us, thus allowing us to see it as it was in the past.
Yes, the age of the observable Universe has changed over time. As the Universe expands, the rate of expansion decreases. This means that the Universe was younger in the past and will continue to age as it expands.
Our current estimate of the age of the observable Universe is considered to be accurate within a few hundred million years. However, as our technology and understanding of the Universe improves, this estimate may be refined and become even more precise.