Questions tagged [density]

Questions on the ratio of an object's mass with its volume.

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Luminosity distance in a FRW Universe

In the following presentation: https://www.slac.stanford.edu/econf/C0307282/lec_notes/pinto/pinto.pdf we have the relation for the luminosity distance in a FRW universe: \begin{aligned} D_L & =\...
Vincent ISOZ's user avatar
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Newtonian theory of small perturbations: Continuity equation

Starting with the continuity equation $$\frac{\partial \rho}{\partial t} + \nabla_r(\rho \vec{u}) = 0$$ making those substitutions. $$\nabla_r = \frac{\nabla_x}{a}$$ $$\rho(\vec{x},t) = \bar \rho(t)[1 ...
merlinbluepickle's user avatar
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Fourier transform of the density fluctuation

There is a Fourier transform that I don't really understand in my textbook (p.218). I have the following equation: $$\ddot{\delta} + 2H\dot{\delta} -\frac{3}{2} \Omega_m H^2 \delta = 0 $$ Then using ...
merlinbluepickle's user avatar
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If a rogue planet entered near Jupiter planet could it tear the planet apart?

Theoretically, there could be a rogue planet made of iron or even osmium in large quantities, which can make it have 25 Earth masses within the volume of 1 Earth Radii. So if this object came near ...
MiltonTheMeme's user avatar
2 votes
3 answers
300 views

Is there evidence against a Universe Black Hole?

As far as I know, the critical density of our observable universe coincides with the density of a huge black hole of the same radius. It could be a BH expanding at the speed of light and growing in ...
Juan Casado's user avatar
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Question about the core mass fraction of planets and its relation to the magnetic field

I have a question about planetary cores and their relation to the magnetic field. Around how massive (using this in the sense of mass, not size, for clarity) does a core need to be in relation to the ...
DanceroftheStars's user avatar
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Consistent values for density of galaxies between degree squared and steradian

I have a table of densities of galaxies : Expected number density of galaxies for photometric survey per unit area and redshift intervals, $\mathrm{d} N / \mathrm{d} \Omega \mathrm{d} z\left[\mathrm{...
guizmo133's user avatar
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Is the object mass distribution different for galaxies of different diameter?

Let's say we compare two elliptic/spiral galaxies with the bigger having a diameter 100 times the smaller. Now we count every separated object inside them and classify them into mass categories e.g. $...
J. Doe's user avatar
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How to plot the freeze out equation for massive particle species in Mathematica? [closed]

I want to recreate the figure . This comes from the Cosmological Boltzmann equation which can be modified as the number density equation as $$\frac{dY}{dx}=-\frac{s(m)<{\sigma}|v|>[y^2-Y_{eq}^2]}...
Trinayan Saikia's user avatar
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Besides Luna, are there any models of how moons become spherical and round?

Question Given the theorized exceptional formation of Luna, (Giant-impact Hyphothesis, Wikipedia), are there any similar animated models of how the other, much smaller, moons in our solar system ...
elika kohen's user avatar
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What is the sharpest stellar number density variation known?

In the solar neighborhood the stellar number density is about 0.14 stars per cubic parsec. This changes with a length scale of 220-450 pc as we move up or below the Milky Way disc, and 2.6 kpc ...
Anders Sandberg's user avatar
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If the ratio of actual total mass to critical mass (Ω) in the universe is only about 0.3, why do cosmologists believe it is flat, as far as we see?

What about astronomical observations makes scientists believe our universe is flat, at least as far as they can tell? Despite the critical Friedmann density being less than one, plus the existence of ...
Kurt Hikes's user avatar
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Does the density of a black hole decrease after releasing a large amount of Hawking radiation?

Is is true that the smaller a 'regular', stellar-mass black hole is, the denser it is inside of its event horizon? After all, if you look up (or calculate) the Schwarzschild radii of the Sun and the ...
Kurt Hikes's user avatar
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What do "overdense" and "underdense" mean in cosmology?

“Overdense" and "underdense" often appear in various documents to refer to high density and low density respectively. And I know that "overdensity" is defined as $$ \delta(x)=\...
Wang Yun's user avatar
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Energy conservation of the universe

I have tried to derive an equation for the total energy of the universe. I have found that, $$E(t)= \delta\dot a(t)^2a(t)\Omega(t)$$ Where $\delta$ is just a positive constant, a(t) is the scale ...
Stan's user avatar
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Aren't denser bodies more likely to collapse into hydrostatic equilibrium?

It is said that at small sizes, icy planets are more likely to be in hydrostatic equilibrium than terrestrial rocky ones. But why, as a matter of fact? Shouldn't denser bodies be more likely to ...
John's user avatar
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Looking for analytical expression of Cosmic Variance - Poisson distribution?

I have an expression of Matter Angular power spectrum which can be computed numerically by a simple rectangular integration method (see below). I make appear in this expression the spectroscopic ...
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2 votes
4 answers
304 views

If a black hole has the same mass as me, then why dont i suck everything at my center of mass?

So, a black hole is very dense that you can get really close to its center of mass that it has a strong pull, well, if i have the mass of a black hole that has a really small event horizon, why doesnt ...
Programmer's user avatar
6 votes
2 answers
248 views

Are any supergiants translucent?

Are any supergiants translucent? Some have volumes thousands of times more than the Sun's while having maybe twenty times the mass of the Sun which makes them sound rather diffuse. If there was a very ...
Some Student's user avatar
8 votes
1 answer
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What is the history of the average pressure, density, and temperature of the matter in the universe over time?

This question is inspired by this more specific question where Cerelic wanted to know if conditions were suitable for liquid water to exist during an epoch when the characteristic temperature of the ...
Mark Foskey's user avatar
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Did the density wave theory for spiral arms take account of this phenomenon?

I came across this link. In it the author speculates that in the outer region of a galaxy, the spiral arms overtake the stars, and vice versa for the inner region. That means there must be a middle ...
longtry's user avatar
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Is is possible for a main sequence star to have 0.77 solar masses yet only be 0.54 solar radii?

I know ordinarily a star's mass and radius tend to be proportional, so a 0.5 solar masses star tends to be about 0.5 solar radii and have a density of 4.82 g/cm³, 3.42 or about three and a half times ...
Xi-K's user avatar
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What increases or decreases the mass and density of a fixed radius star?

Assuming a relatively even proportion of mass and radius, a 0.25 solar masses and radii star would have a density of 22.5003 g/cm³, or about 16 times our Sun's density. Keeping the radius of 0.25 ...
Xi-K's user avatar
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What factors influence a star's temperature and density?

Assuming a relatively even proportion of mass and radius, a 0.25 solar mass and radius star would have a density of 22.5003 g/cm³, or about 16 times our Sun's density. However, there is obviously a ...
Xi-K's user avatar
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6 votes
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Are red dwarfs really 30-100 times our Sun's density?

This article by K. E. Edgeworth claims red dwarfs tend to be around 30-100 times our Sun's density. However, this seems a bit high. With smaller stars, mass tends to be proportional to radius but ...
Xi-K's user avatar
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4 votes
1 answer
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How do pressure, temperature, and gravity affect a smaller body than Earth with the same bulk composition?

Earth has a density of about ${\rm 5.513 ~g/cm^3}$ with one Earth radius and one $g$. Venus, a very similar planet to Earth and likely with about the same bulk composition, has a density of about ${\...
Xi-K's user avatar
  • 403
2 votes
1 answer
200 views

Demo to get Matter Power Spectrum in cosmology

I would like please to demonstrate the expression of Power spectrum in Cosmology : First, I have the relative contrast: $$\delta_{i}(\vec{x}, z) \equiv \rho_{i}(\vec{x}, z) / \bar{\rho}_{i}(z)-1\quad(...
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How is the density parameter estimated via cosmic microwave background anisotropies?

In 2003 and again in 2018, the density parameter was estimated based on measurements of the cosmic microwave background anisotropies. Can someone please explain how that works? Why do the anisotropies ...
Casper's user avatar
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Historical estimates of the density parameter

From the reference in this answer I learned that our current estimate for the density parameter (i.e., the density of the universe divided by the critical density, which determines the shape of the ...
Casper's user avatar
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1 vote
1 answer
471 views

density of dark matter halo

I was surfing the net for knowing the actual density of dark matter halo for a particular Mh(Mass of the halo) and finally, I got two equations: for pseudo-isothermal halo and by NFW-profile So, ...
Nitin Kumar Chauhan's user avatar
2 votes
2 answers
1k views

How to calculate the orbital period and the density of a planetary body?

I'm a student studying in 9th grade, I am making this fictional habitable planet named "Darwin B" for a planet making competition. It orbits a sun-like star at a distance of 1.15 AU or 172 ...
Ansh Saxena's user avatar
2 votes
2 answers
410 views

Can a high enough density ignite a gas giant into a brown dwarf?

I wonder whether the nuclear fusion of brown dwarfs and more massive stars really depends on their mass only or if it could also start nuclear fusion if it is dense enough but not as massive as brown ...
Ioannes's user avatar
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Density of a sunspot compared to the surrounding photosphere?

Is the density of a sunspot different by more than a factor of ten compared to the area surrounding the sunspot?
Bob516's user avatar
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14 votes
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At the Big Bang, when everything was close together, why did it not "collide", violating Planck length or Pauli Exclusion Principle?

How could so much matter, or "all" in fact, have been concentrated in a smaller universe without being in the actual same place? Why did this not result in undercutting the Planck Length or ...
nilshi's user avatar
  • 259
5 votes
1 answer
160 views

Interpretation and understanding of the relation for the photometric redshift in a given bin

In the context of photometric probe of surveys (like LSST), I need to understand the relation I have to use for photometric bins. Considering $p_{ph}(z_p|z)$ the probability to measure a photometric ...
user avatar
14 votes
3 answers
5k views

Are there any planets or moons denser than Earth?

Earth has the highest density out of all planets, planetoids and moons of our planetary system, and also has a higher density than the Sun. Do we know any exoplanets or moons denser than Earth?
user30007's user avatar
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1 vote
1 answer
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Factors included in the average density of planets

When the average density for a planet is stated does it incorporate the mass of the atmosphere, when a planet has an atmosphere, or is the mass of the atmosphere ignored?
Fred's user avatar
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Is the atmosphere of neutron star solid, liquid or gaseous state? [duplicate]

Neutron star has a thin layer of atmosphere consist mostly of hydrogen and helium and is about less than a metre thick on average, but what wasn't stated in many articles I read is the state of matter ...
user6760's user avatar
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3 votes
1 answer
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If interstellar medium was dense plasma and light slowed down, would things appear to move slower far away?

Voyager 2 recently left the solar system and registered an increase in plasma density. If light moved slower in this dense plasma, would it appear like events in other solar systems played out slower ...
chron0's user avatar
  • 31
2 votes
1 answer
177 views

What is the least dense exoplanet?

An exoplanet with density 0.31 grams per cubic centimeter has been found. Is this the least dense exoplanet we know of?
usernumber's user avatar
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5 votes
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Can a planet exist with the same mass as the earth, but have a different diameter?

Can there be a planet with the same mass, and a similar composition, as the earth, but with a larger or smaller diameter, and how could that theoretically occur? I've tried to see if there is any ...
DrakeShade's user avatar
54 votes
5 answers
11k views

Is there a star over my head?

Say I'm standing up straight, and I draw a straight line from my core through the top of my head (perpendicular to the ground). What is the probability that that line intersects with a star? EDIT: I'...
user68873's user avatar
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4 votes
0 answers
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Cosmology : Formula for the bias of galaxies

first I have posted originally on the physics stackexchange but I have not had answers about my question, so I try my luck on this specific forum (if forums leaders want to delete the original post on ...
user avatar
2 votes
1 answer
1k views

How to find inverse steradian from $\text{arcmin}^{-2}$ for density of galaxy

I am using a code on EUCLID future mission. The original author of this code has set a value for the density of galaxy equal to : ng = 354543085.80106884 I ...
user avatar
3 votes
4 answers
2k views

Black hole density

How does more compression relate to a stronger gravitational pull. Like, when we say that a black hole is a tiny space that has 20-30 suns compressed in it, how does this increase its density and ...
penguin99's user avatar
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7 votes
1 answer
1k views

what determines the density of atmospheres

What factors determine the atmospheric density on various planets and moons? Some larger bodies have less dense atmospheres, while some smaller ones have a more dense atmosphere.
John Canon's user avatar
2 votes
1 answer
130 views

From original burst, fraction of stellar mass still surviving on Main sequence

Suppose that all stars in this galaxy were born in a single major-merger burst event about 10 Gyr ago. From this original burst, I want to compute the fraction of stellar mass still surviving as stars ...
user avatar
1 vote
1 answer
99 views

number density of Dark Matter Halos Versus number density of Galaxies

Can we count only one dark matter halo for each Galaxy ? Indeed, I am trying to estimate the total density of galaxies in given volume and I have often seen on physics exchange and other forums the ...
user avatar
3 votes
1 answer
1k views

Why the forbidden lines of [OIII],[NII] and [SII],[OII] are sensitive to electron temperature and electron density, respectively?

I am trying to understand the topic of "Metallicity estimates", either HII regions or planetary nebula. For this latter, the electron temperature (and density) must be known. I am studying the ...
Luis Enrique Garduño Puga's user avatar
1 vote
1 answer
265 views

Are there any known asteroids with average density similar to that of Earth's?

In this answer I mention that for very low orbits around spherical bodies, the period tends to scale only as the inverse square root of the density, and not the diameter. For a low orbits where the ...
uhoh's user avatar
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