# Tag Info

8

Here is simple explanation: Suppose a light source and a observer are in an expanding space. Now think of two subsequent crests of wave emitted by the lightsource. The second crest is emitted slightly later than the first one, hence the space has expanded slightly in the meantime. Consequently, the second crest has to travel further to reach the observer, ...

5

No. Evidence for the accelerating expansion of the universe comes from multiple angles: supernovae data, Baryon acoustic oscillations, the mass functions of galaxy clusters, etc. That the universe's expansion is accelerating is not in doubt; the question is by how much.

4

The situation of a geometrically open universe (i.e. one with $k=-1$) is dealt with from p.24 of these lecture notes. Fora "matter-dominated" universe (i.e. one where the energy density of matter is dominant, $\Lambda=0$), which appears to be what your sketch illustrates then we can use $$a^3 \rho = a_0^3 \rho_0 = \rho_0$$ The first of Friedmann's ...

4

The simple answer is that we don't know that dark energy is uniform on any but the largest scales. The only evidence we have for the existence of dark energy comes from: the Sn1a supernovae light curves the measurements of the cosmic microwave background Both of these measure on scales far larger than galaxies. The resolution is more like 100 to 1000 ...

4

They're related, but not exactly the same: Our past light cone is the part of spacetime from which light has had the time to reach since the Big Bang. The observable Universe is the part of space from which light has had the time to reach since the Big Bang. It is bound by the so-called particle horizon. Note the subtle difference; our past light cone ...

4

The turning point corresponds to the distance where the rocket would just about take infinite time to reach the Earth again. This can be calculated using a cosmological model that describes how the scale factor $a(t)$ changes over time. The model is defined using estimates of the cosmological parameters such as matter and energy density and types plus the ...

3

here is a layman's explanation: There is a toy called a Slinky which is a loosely-wound coil spring made of either plastic or flat wire- if you haven't seen one, search on it to get the idea of how it looks. We imagine the Slinky resting horizontally on a smooth floor, with its ends pulled apart to some convenient distance so adjacent coils of the spring ...

3

When we are doing a calculation in general relativity we usually have to choose some coordinates, and one of these will be the time coordinate. The time measured using our coordinates is then called the coordinate time. It is important to understand that the coordinate time is just a label we use to identify points in spacetime, and it need not and ...

3

There is a potential tension between low redshift probes of mass clustering and Planck data (CMB measurements). This ongoing speculation might be evidence of new physics or even modifications of general relativity. However, the author of the article you cited seems to have a confusion between the cosmological constant (no tension discussed in the literature) ...

2

The layman explanation of the expanding universe is a balloon. But now you have to imagine that observers behave like points on the balloon that do not grow like the waves on the balloon do. More technically, it should be noted that not only doesn't the experimenter that is measuring the red shift remain unaffected by expansion of the universe but also the ...

2

You are correct in stating that the age of the universe depends on the observer/frame of reference. This is unavoidable because of the theory of relativity. However, when it comes to discussing cosmology there is a preferred frame of reference that makes sense to consider. One of the cosmological assumptions is that the universe is homogeneous and isotropic....

2

In cosmic inflation, the Friedmann scale factor $a(t)$ of the universe doesn’t grow as some power of the time, such as $a\sim t^2$ or $a\sim t^{20}$. These are not exponential expansions at all. Exponential expansion means that time appears in the exponent: $a\sim e^{t/\tau}$ or $a\sim 2^{t/\tau}$ or $a\sim 10^{t/\tau}$ or whatever you prefer to use as the ...

1

Your idea would not work. Once the signal has reached a station, the station does not need to re-send the signal, because the original signal is already moving with the speed of light relative to the station. Thus the presence of the station does not increase the speed of communications. When a distant signal reaches a station, the station would already be ...

1

It is not directly because of dark energy (he said dark energy, not matter), it is because of the accelerating expansion of the universe, which is believed to be caused by dark energy. The distance between opposite sides of the universe is increasing faster than the speed of light, so the light can never reach the other side. Any space crafts sent out would ...

1

Cosmologists believe the universe is expanding at an accelerating rate because the measured value of the cosmological is positive. The "tension" is that the two different methods give different values of the Hubble parameter (from which the cosmological constant can be calculated). There is a low probability ($4.4 \sigma$ according to Wikipedia) that these ...

1

You need to remember Hubble's discovery - the farther away an object is, the more the light it emitted has been redshifted between it and you. Because light moves at finite speed, this also means that the time between when the light was emitted and observed also correlates with redshift. So, when someone says something "has a redshift" or is "at a redshift", ...

1

As far as I know (not an expert), the constant energy density you are referring to is the "dark energy" and its density is inferred from cosmological observations and inserted into Einstein equations as a constant parameter. That is, it is not measured directly but is "fixed" by assumption to be constant such that expansion of the universe will be consistent ...

1

One situation I've found conformal time very useful in in the context of spacetime diagrams. Since the ratio between proper time $t$ and conformal time $\eta$ is the same as the ratio between proper ("physical") coordinates $d$ and comoving coordinates $\chi$ — namely the scale factor $a$ of the Universe — spacetime diagrams showing conformal time as a ...

1

I think I figured out the answer. There's not a lot of difference between the conformal time and chronological time in an analysis of the time leading up to recombination. After all, it's just a coordinate system and there's a simple translation between one coordinate system and the other. Pah-tay-toe, Pah-tah-toe. As nearly as I can tell, the primary ...

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To throw added confusion into the mix we have this result based on measurements with red giant stars. The use of luminous red giants leads to $H=70\ \mathrm{km/(s\ Mpc)}$ which is in between the CMB and Cepheid variable meter stick results. This suggests we may really have a problem with getting different meter sticks to calibrate together in a consistent ...

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