# Tag Info

Accepted

### Negative Horizon distance

The integral $$\int_0^t\frac{\mathrm{d}t^\prime}{a(t^\prime)}\propto \int_0^t\frac{\mathrm{d}t^\prime}{t^{\prime \frac{2}{3+3w}}}$$ diverges if $w\leq-1/3$. It doesn't give a negative result (nor ...
• 6,003
Accepted

### Age of universe vs Hubble time in Milne universe

At $t = t_0, \, a(t_0) =1$. Thus, $t_0$ is the time when the scale factor is unity. If you want to assume that the scale factor is one right now, then $t_0$ is today. To measure expansion rate, I ...
• 2,515

### According to Hubble's Law, how can the expansion of the Universe be accelerating?

So it tells you what the recession velocity of a galaxy is right now, not what it was in the past. Measurment is NOW and event ("acceleration") is in the PAST.
1 vote
Accepted

### How to find critical density?

The Friedmann eqn is: $$H^2+\frac{k}{a^2}=\frac{\rho}{3M_p^2}$$ where $M_p^2=1/8\pi G$ is the Planck mass. In this eqn we take cosmology constant as dark energy, which becomes a part of $\rho$ above. (...
Accepted

### How to understand critical density?

1: The critical density is the required density for a flat universe, so it requires only $\Omega_{\rm K}$ to be $0$, but not $\Omega_{\Lambda}$, so it also holds with dark energy if you include its ...
• 12.2k
1 vote

### Why are spherical shapes so common in the universe?

In a wider sense: The sphere is the smallest surface that encloses a given volume. The sphere is the solution to one of the most general optimization problems in three-dimensional geometry. This makes ...
1 vote

### In a universe with no photons, will everything necessarily be at absolute zero temperature?

Once you remove photons (and so, incidentally, remove the electromagnetic force) then you have a very strange universe in which all fundamental particles are neutral and there are no inelastic ...
• 55.7k

### In a universe with no photons, will everything necessarily be at absolute zero temperature?

A universe filled with only dark matter would contain no photons at non-zero temperature. It requires matter that is coupled to the electromagnetic field to generate radiation.
• 24.9k

### Why are spherical shapes so common in the universe?

The surface to volume ratio is the least for a spherical object. This means that a sphere for attractive forces will have the highest binding energy. All objects tend to a state of lowest potential ...
• 1,395

### In a universe with no photons, will everything necessarily be at absolute zero temperature?

Temperature is just another word for "mean kinetic energy". Not being able to radiate heat would certainly lead to strange phenomena such as a heated body retaining its temperature ...
• 1,337

### Why are spherical shapes so common in the universe?

I kept reading and reading and couldn't believe I wasn't finding anything about hydrostatic equilibrium.... Until I did. In space, when an object has enough mass, it will ALWAYS take the shape of a ...
Accepted

### R-W Metric and null geodesic path of photon

In any metrics in general relativity, the photon geodesic is light-type by definition. Consequently, the scalar product of an elementary displacement of the photon \$ \overrightarrow{ds}.\...

### In a universe with no photons, will everything necessarily be at absolute zero temperature?

Rather than a lack of photons, you could consider inside a black hole where photons just don't move. Motion leads to heat. In such a case it would be very close to absolute zero in a one-solar sized ...
• 4,715

### Why are spherical shapes so common in the universe?

Although Torus/Donut Shapes are among the most common shapes in the transparent, invisible universe, like the Magnetospheres of various planets and galaxies, what humans can easily discern are dense ...
• 121

### Why are spherical shapes so common in the universe?

A planetary body would always want to achieve hydrostatic equilibrium: In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at ...
• 1,257

### Why are spherical shapes so common in the universe?

Equilibrium states in time-varying processes are local minima of exergy which preserve conserved quantities. Those things are (or can be reliably predicted to be) roughly sphere-like for which ...
• 13.9k

### Why are spherical shapes so common in the universe?

On a astronomical scale spherical shape is due to gravity and the more massive the body the bigger gravity. Let's start with the stars: Stars form by collapsing cloud of gas in so called "pre-...
• 426

### Why are spherical shapes so common in the universe?

A classical philosopher like Aristotle would say this is because "sphere is a perfect solid and the heavens are a region of perfection". A more modern version of the essentially same answer ...
• 3,772
Accepted

### Why are spherical shapes so common in the universe?

Spherical shapes in the universe are common because the dominant long range forces like gravity and electromagnetism are central (in that they only depend on the distance between objects). Our planet, ...
• 2,687
1 vote

### Can glueballs and bosons survive indefinetely in space (forming structures)?

Can <…> survive indefinitely in space (forming structures)? No, in a universe with positive cosmological constant. Whatever the mechanism is for formation of a composite object (that you called ...
• 15.8k

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