# Tag Info

3

This is yet another instance of taking the ubiquitous balloon analogy too far. See, while it's a wonderful way to express the expansion of the universe, there are some misconceptions that arise from it: We live in a universe of finite size (we don't know, but we think not) and non-zero curvature (according to WMAP, we don't, or at least we think we don't) ...

0

If you mean by "our universe" the matter in spacetime we are able to reach and observe then you are right. The universe will become more and more finite for us unless someone will invent a "warp drive" or "wormhole" (currently the probability for it is very low). According to research you have ca. 100bn years time before all others galaxies will be gone ...

2

Galaxies are not moving away from us, it is the space between us and the galaxies (and everything, in general) that is continually expanding. This is allowed to happen faster than the speed of light, because no object actually crosses the light speed barrier in the process. So consequentially, the universe has no size constraint like the one you've stated.

1

As pointed out in the answer above, the observations that lead to the dark energy theory were not all distant (7-14 billion light years), but less so. Dark Energy expansion is observed throughout much of the observable universe - not just the very distant. Also, consider the basic hubble discovery - galaxies 4 billion light years away were moving way from ...

5

The thing is, we don't completely base our understanding of the expansion of space on galaxies 7 to 14 billion light-years away. For evidence that the universe is expanding, look at Edwin Hubble's original paper in which he confirmed what we now call Hubble's law. The galaxies he studied are on the order of millions of parsecs away. Multiply that by 3.26 to ...

-1

There is a concept known as entelechia, which basically means an idea so complicated and so full of border cases that no useful ideas can be obtained from it. One entelechia is the concepto of god, a súper being who created everything in just 6 days and who needs your money. And the fossil record was put there by good just to test your faith. Eventually it ...

3

The width of a SN light curve is changed due to a time dilation between the source and the observer. If the source emits light with wavelength $\lambda_\text{em}$, it will be observed with wavelength $\lambda_\text{ob}$, so that its redshift is $$1 + z = \frac{\lambda_\text{ob}}{\lambda_\text{em}}.$$ We can also write everything in terms of frequencies ...

0

>> How confident are we that mass is not being lost in the universe? << * Mass (energy) can be lost in principle: if you convert mass to radiation (which you can, because mass and energy are equivalent), the radiation density dilutes with the growing scale factor to the 4th power because of the redshift, while mass density only dilutes with ...

0

You ask I'm struggling to remember my undergrad physics - would two particles each with an initial velocity moving away from each other in a gravitational field (relatively) speed up if the gravitational field is reduced? To answer this part of your question, gravity only works in one way, pulling things together, never allowing them to move away from each ...

4

There is an expectation that the redshift of an object will change with time. The details depend on the cosmological parameters (a plot is shown below). This was first explored by Sandage (1962) who predicted that, in a matter-dominated universe (i.e. no consideration of dark energy back then), the redshift should decrease due to the braking action of ...

2

Cosmological red-shift depends on the difference in the scale factor between when the light was emitted and when it was received and not directly on recessional velocity, which is of dubious physical significance. The recessional velocity of any given galaxy outside our own (ignoring peculiar velocities) in a LCDM-like cosmology will be arbitrarily large ...

5

You are not missing anything, apart from a slight misinterpretation of the type Ia supernova cosmology results. With no dark energy, the rate of expansion of the Universe would slow down. The gravitational "pull" of everything would be responsible for this de-celeration. When we look at a distant galaxy, we see it as it was when the light was emitted. ...

-2

The idea that space is expanding is a relation between it's size and time. Time expanding would be a relation between time and time, so it can't???

4

It's really an either/or proposition, i.e. either space is expanding or the time experienced by distant objects is dilated, depending on how you view the situation. We choose the former description because it is better. To expand (excuse the pun!) on what I mean, the measurable result of time dilation is red-shift and indeed distant cosmological objects ...

0

You ask Is there more time, longer time or is the time part not affected at all by expansion? First off, Is there more time? I don't know because I dont know exactly what time is? Do you? Did Einstein? No, I think he said he didn't in one of his books? Does anybody? Probably not. Is there longer time is an easier question because I don't think that ...

4

The CMB was emitted from everywhere, in all directions. The CMB emitted at the point where you are standing right now, has now been dispersed to a distance $d_\mathrm{CMB}$ equal to the distance that light can travel in the almost 13.8 billion years that have passed since it was emitted*. (note that $d_\mathrm{CMB}$ is much larger than 13.8 billion ...

2

Cosmology models the data for the creation of the universe with the Big Bang model.. The BB has a beginning as a singularity, and in the early times there was no "flatness". The model is a four dimensional model in space time. All (x,y,z) points in our three dimensional neighborhood were in the original singularity; thus all points are at the center of the ...

1

Yes you are correct Sophia. This is a major component of the standard model itself, which concludes the opposite (that space is expanding in an infinite cosmos), but the only reason a conclusion like that comes about, is because what we observe is indistinguishable from being at the centre of a finite universe that races away from precisely us, uniformly in ...

3

Yes. The Milky Way is a very old galaxy, having formed roughly half a billion years after the Big Bang. So if we observe a galaxy that has a redshift of ~10, we are looking back in time to approximately this epoch, so an alien astronomer in that galaxy observing the Milky Way today would see it redshifted by the same factor, and would observe it in the ...

1

If the Maxwell equations are valid up to the Galaxy of which you talk, and all the time, then the answer is "yes", because these equation are symmetric in time. That means, the trip of the light from the galaxy to us, is reversible in time, i.e. occurs also in reverse. It's simple to show that the time symmetry is equivalent with rolling the movie in ...

3

The sodium line that is being referred to is a reasonably sharp, dark absorption feature that is seen in the orange part of the visible spectrum of the bulk of stars that make up the light from a distant galaxy. The absorption is caused because stars are hotter in the middle than they are on the outside. The relatively cool gas on the outside has sodium ...

1

It is a standard line in the atomic spectrum of sodium, that is used as a standard The line you refer to in nmeters is a standard line because of its strength for determining refractive indices of materials in the lab. A reference wavelength of 589.3 nm (the sodium D line) is most often used. T The evolution of the line due to the motion of the ...

1

The emission spectrum of sodium contains two very bright yellow lines called the D lines. Since they are so bright and so easily seen they are something of a favourite for spectroscopic studies. In this case the question is referring to the D1 line, which has a wavelength of 588.9950nm (the D2 line at 589.5924nm is only half as bright as the D1 line). The ...

0

There is, in the Alekseev paper a rather strange description of the dark matter. In introduction he writes: 'It was postulated that the source of antigravitation is “dark matter” which inferred to exist from gravitational effects on visible matter'. The search for the dark matter was triggered by the observations showing the conventional gravitational ...

Top 50 recent answers are included