New answers tagged

2

Dark matter is gravitationally ordinary, as G. Smith says, and attracts. Dark energy can be understood as a combination of energy-density and tension, somewhat as a stretched elastic band has tension. According to general relativity, energy-density always attracts gravitationally, and the presence of tension always counteracts this. The tension in ordinary ...


1

The gravity of dark energy is repulsive (but not because it has negative mass or negative energy density). However, the gravity of dark matter is boringly attractive. The difference between dark matter and ordinary matter is not in their gravity but in their electromagnetic interactions: dark matter doesn’t have any, so it doesn’t emit or absorb light or ...


2

The answer is yes: they are related. The quantum field theory estimate is wrong by some huge factor (e.g. 120 orders of magnitude) but this could be avoided if it were possible to simply assert that the cosmological constant $\Lambda$ is zero. But the astrophysical observations suggest $\Lambda$ is not zero. In a bit more detail, the situation is as follows....


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 ...


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, ...


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 ...


0

The red-shift does not come from direct intervention from metric expansion itself, but from regular Doppler shift, albeit due to the separating velocity between source and detector, which depends itself on the distance between source and detector through Hubble's law Just for completeness, I'll mention that the original hypothesis that red-shift came from ...


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 ...


0

If it was varying over a period of billions of years, we'd see variation in the spectrums of distant galaxies, and we do. Is this a possible implication of universal red-shift? Or even an alternative to the expansion explanation? This has been explored before in both scientific and creationist lines. The problem is that we can see the spectra ...


0

Probably not- here is one explanation. When we look at the most distant objects in the universe (super-bright galaxies or active galactic cores) we are seeing them in an early stage of their lifetimes. It is thought that at those times, the supermassive black holes in those galaxy centers were in the process of actively gobbling up dust and gas in their ...


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 ...


0

When you write "point" in scare quotes, what you're essentially doing is reinventing the notion of boundary constructions. A couple of good surveys on this topic are: Sanchez, "Causal boundaries and holography on wave type spacetimes," http://arxiv.org/abs/0812.0243 Ashley, "Singularity theorems and the abstract boundary construction," https://...


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 ...


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 ...


0

The proper age of the universe is the one that is measured the longest out of all frames of reference. It can be shown that it is the comoving time. Thus the comoving time is the age of the universe not as an arbitrary or convenient choice, but because it matches the proper age. In the $\Lambda\text{CDM}$ cosmology, the Friedmann equations have an analytic ...


4

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 ...


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....


0

I think this question has may have been answered before, but perhaps not in the way you are asking. From the perspective of our experience we consider that the dimensions of space and time are fixed but relativity tells us that this is not so. Both space and time can bend (as you clearly understand). So when our space and our time are not uniform how do ...


0

In the quote it should be "causal physics" not "casual physics" In this context cosmologists have sometimes used the term "horizon" for "Hubble distance" whereas strictly they are different concepts; you should check what is the usage in the book you are using. Both are relevant. There are two physical observations that are relevant here. First, in order for ...


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 ...


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 ...


0

How could someone inside the spacetime measure the expansion of the time dimension they participate in? It could expand all day long but to the interior time is constantly flowing.


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) ...


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.


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 ...


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 ...


Top 50 recent answers are included