# Questions tagged [density]

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

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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(... 2 votes 1 answer 29 views ### 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 ... 3 votes 1 answer 124 views ### 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 ... 1 vote 1 answer 363 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, ... 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 ... 2 votes 2 answers 378 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 ... 3 votes 0 answers 101 views ### 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? 14 votes 1 answer 4k views ### 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 ... 5 votes 1 answer 156 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 ... 13 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? 1 vote 1 answer 93 views ### 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? 2 votes 0 answers 41 views ### 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 ... 2 votes 1 answer 58 views ### 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 ... 2 votes 1 answer 154 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? 5 votes 2 answers 1k views ### 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 ... 55 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'... 4 votes 0 answers 100 views ### 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 ... 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 ... 3 votes 4 answers 1k 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 ... 7 votes 1 answer 902 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. 2 votes 1 answer 106 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 ... 1 vote 1 answer 89 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 ... 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 ... 1 vote 1 answer 227 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 ... 8 votes 2 answers 304 views ### Can "rogue" supermassive black holes be made this way? Could two galaxies (one big and one small)intersect at a velocity to allow the smaller black hole to escape but not the galaxy around it? -5 votes 2 answers 643 views ### Why isn't the Sun hollow? [closed] [Edited to remove incorrect geological assumptions] The sun is self-rotating, i.e. it spins on its axis. When something spins, there's an apparent force away from the centre, and matter gets flung ... 4 votes 2 answers 152 views ### Does Flat Universe mean even distribution? I've read through all the other related answers to "flatness" questions but I need a bit more clarification. I understand that a triangle in a 2D universe would not equal 180 degrees in both a closed ... 6 votes 1 answer 187 views ### Does the sun blow a bubble into the interstellar medium? I once read that the solar wind is more or less blowing a "bubble" into the interstellar medium. However, the numbers I found while trying to verify this don't give me a clear picture yet: The ... 3 votes 2 answers 147 views ### Question about 3p/c^2 in Friedmann equation The Friedmann Equation:$$\frac{\ddot{a}}{a} = -\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3} I want to know, where does the $3p/c^2$ come from. I know it is something ...
Let universe be completely made from hydrogen. And also we have redshift $z= 6$. with Hubble constant $H_{0} = 2.1941747572815535\times 10^{-18}\:\mathrm{s}^{-1}$. We also know that density of the ...
I'm trying to get an equation for the energy density of matter of the universe $\rho(t)$, assuming the number of particles is conserved and rest mass energy is much greater than kinetic energy. \$\rho(...