# Tag Info

Accepted

### How is the Universe's Expansion Accelerating if the Hubble Constant is Decreasing?

The Hubble parameter is defined as the rate of change of the distance between two points in the universe, divided by the distance between those two points. The Hubble parameter is getting smaller ...

### Is there any proof of space being created?

This is an intriguing proposition, but I would ask how your hypothesis explains that the universe appears to be flat? That is with $\Omega_M + \Omega_\Lambda = 1$. The evidence for this comes from ...

### How long has it been since dark energy started to reverse our slowdown and accelerate the expansion of the universe? 4 billion years ago? 4.5? 5?

It is likely that you or your sources are conflating two different events. Dark energy begins to comprise the majority of the Universe's energy density about 4 billion years ago. The expansion of the ...

### Can the diminishing energy of the CMBR be the source of dark energy?

No - the decreasing energy in the CMB is already well modeled in the Friedmann equations. The term in the density parameter that is proportional to $a^{-4}$ is the contribution of radiation energy ...
Accepted

### Will gravitational waves too far away ever reach us?

The answer here is very similar to if you were asking about light. In principle gravitational waves might allow us to fractions of a second after the big bang. Electromagnetic waves can see back to ...
Accepted

### What are GEODEs?

Disclaimer: I'm Dr. Kevin Croker, lead author on the ApJ series in question. I work on formal aspects of relativistic perturbation theory. I think the best way to answer your question is to just ...
Accepted

### Why would a quantity like the 'Hubble contrast' be squared, then have its square root taken?

The brackets refer to the average, so $\left< x^2 \right>^{1/2}$ is the root-mean-square (RMS) of $x$. That is the square root of the mean (or average) of the square of multiple $x$s. The RMS ...
Accepted

### Why can't dark energy be considered a 5th fundamental force?

Dark Energy is a scalar field, whereas forces are vector interactions As far as we know, we are sure that dark energy, or more accurately, the expansion of space-time, is a scalar quantity. That is, ...
Accepted

### Does the universe expand at the same rate everywhere in the universe?

What's outside the observable Universe, we can't say anything about, but averaged over large enough scales ($\gtrsim$ a billion lightyears), it does indeed seem to be expanding uniformly. However, ...

### Why can we trust Hubble Time if the rate of expansion is not constant?

$H_0^{-1}$ is only a rough estimate for the age of the universe and you have correctly identified the reasons why not. A correct age estimation relies on knowing $H_0$ and the densities of matter and ...
Accepted

### Relationship between Dark Energy and Dark Matter

First of all, if there were no dark matter (DM), you wouldn't ask this question, since structures — including galaxies, stars, planets, and you — wouldn't have had the time to form in the early ...

### Can the Dark Energy be Pauli pressure?

There are several reasons dark energy cannot be pressure due to the Pauli exclusion principle. First of all, pressure does not cause expansion of the universe, because pressure is not a force-- ...

### If objects don't move when the universe expands, how can the expansion result in redshift?

The redshift is indeed not caused by the movement of the objects, but by the expansion itself. This is a theoretical result from considering the FLRW metric for a light ray traveling on a null ...
Accepted

### How exactly will DESI simultaneously capture individual spectra from 5,000 galaxies using optical fibers?

You can probably get most if not all of your questions answered by perusing the main DESI web site, which I encourage you to check out. There is, for example, a nice video describing the assembly of ...
Accepted

### When will all other galaxies become not visible from Earth/Milky way?

That will never happen. We will always be able to see the galaxies in the Local Group. We won't get separated from them by the expansion of space because the group is bound together gravitationally. ...
Accepted

### Could Dark Energy be a "Cosmic Gravity Background"

There is in fact a cosmic gravitational wave background. These waves are expected to be stochastic, having originated in the early universe (much earlier than the cosmic microwave background). Random ...
Accepted

### How strong is the gravity from half of the distant universe?

Not very strong at all. I get a rough figure of $3.725\times 10^{-9} m/s^2$. To perform that calculation I made a few simplifying assumptions. Assume that we can ignore everything outside the ...

### Can dark energy or dark matter affect black holes

Dark matter can, and probably does, fall into black holes and affects them just as any other form of matter or energy falling into them does. It doesn't fall in a huge amount because, like anything ...

### How exactly will DESI simultaneously capture individual spectra from 5,000 galaxies using optical fibers?

Supplemental to @PeterErwin's answer, some more details on the five thousand "robots". Each fiber has a circular "patrol area" with a diameter of 12 millimeters, and these are located on a hexagonal ...

### Why can't dark energy be considered a 5th fundamental force?

Dark energy's influence on cosmic expansion isn't a new force. It's just gravity. Calling dark energy a force in the context of cosmology would be like calling the Sun a force in the context of the ...

### Is there any proof of space being created?

As is always the case in physics, there is no proof. But if your scenario were true, it would have to be rather fine-tuned in order to create the observed expansion of the Universe. First of all, ...
Accepted

### Relationship between the Casimir force and dark energy

There are two problems that arise here. One obvious problem is that the Casimir effect is attractive1, while dark energy is repulsive. The other problem is one of scale. Casimir (1948) shows that, ...
Accepted

### How strong is the gravitational stretch we experience from the edge of the universe?

In the past, the universe was very hot and dense, and we are seeing the light from this time period as the Cosmic Microwave Background. This statement is (partially) correct. However.... Since ...

### Why is dark energy density given in grams per cm$^3?$

This is a case of using a sort of natural units (similar to geometerized units), which are often convenient in relativity (special and general). Take, for instance, the Minkowski metric of special ...
Accepted

### What is the closest candidate for Dark Matter currently?

This arxiv post says if there is dark matter in the solar systems, its density should be below $\approx 1.4\cdot 10^{-20} \frac{g}{cm^3}$. They tried to find the dark matter in the solar systems by ...
Accepted

### Are there Dark Stars

Dark matter has different properties than ordinary matter. It cannot collide, and thus it alone cannot coalesce to form a star. Furthermore, dark matter and dark energy are unassociated properties. ...
Accepted

### Has the Big Crunch been ruled out?

The Big Rip happens if the equation of state for the dark energy has $p/\rho = w<-1$, and all empirical data give us $w\approx -1$. A Big Crunch requires a pretty high value of $w$ (it must go ...
Accepted

### Why dimmer high-redshift supernovae means the expansion is accelerating, if the dilated region pertains the distant past?

They are further away than they "should" be according to a decelerating universe model (one without dark energy). That is because the expansion has accelerated. The Hubble parameter isn't a constant, ...