# What is Critical density: Definition and 28 Discussions

In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fission cross-section), density, shape, enrichment, purity, temperature, and surroundings. The concept is important in nuclear weapon design.

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1. ### I Questions about Density Parameter and Critical Density

Hi,I'm reading about critical density and I'm a bit confused about it's derivation.Solving the Einstein equations using the cosmological principle we get the (second) Friedmann equation:  \bigg( \frac{\dot{a}}{a} \bigg)^2...
2. ### I Saddle and flat universe edges?

Hi, I have become very interested in cosmology recently and I had a question about the possible shapes of the Universe. I understand the critical density plays a role in the shape. This is what I know (or think I know). At the critical density of energy and matter, the universe is flat. If it...
3. ### I Flat vs Open Universe: CMBR & Type Ia Supernovae

So CMBR points to a flat universe, and this seems to be the generally accepted model. But in a flat universe is expansion not supposed to slow exponentially, stopping after an infinite time? How does this fit with the observation that distant type Ia supernovae show the universe's expansion to...
4. ### I Does the Universe's Expansion lead to Critical Mass < 1?

We understand that for the Universe to be flat, the critical mass of everything that has mass must total an omega of 1.0. At first, with our lack of knowledge regarding dark energy, we thought that the universe that we detected (ordinary matter and dark matter) did not even add up to a third of...
5. ### Critical density: question correction

Hope I'm in the right section for this question! In the big bang model, the expansion of the universe is slowed down by gravity. If there is enough matter in the universe, then the expansion can be overcome and the universe will collapse in the future. The density of matter that is just...
6. ### Cosmic Invent. HW: Determine Densities & Measurements

Homework Statement One of our homework problems asks us to state the density of Baryons, Cold Dark Matter, Radiation, Dark Energy, and the total density of the universe in terms of the critical density today. It also states to give the density of each quantity in dimensionless Omega units (the...
7. ### What is the Critical Density in the Context of Universe Expansion?

Definition/Summary The critical density is defined to be the density necessary to asymptotically halt the expansion of the universe (i.e. flat or euclidean), slightly less and the universe is 'open' (hyperbolic or saddle shaped), slightly greater and the universe is 'closed' (spherical)...
8. ### Calculating critical density of mass, from Hubble time, for exercise

Jorrie's calculator (Lightcone) makes cosmic history tables which tell you among other things the Hubble times in past years. For convenience let's temporarily use greek Theta Θ to stand for THubble so we don't have to write so much. Basic facts (definitions actually) are that Θ = 1/H and the...
9. ### Find Historical Critical Density Data

I was reading up on critical density, and found the "current" number for it, but can't fine any past records or graphs to show how they have changed over time. Any help?
10. ### Critical Density and the expanding universe

Okay, here's my problem. I've recently read that physicists believe that all the mass and dark energy in the universe do indeed add up to the critical density, which means that the universe will continue to expand forever, at zero curvature, and is either finite (like a torus) or infinite. And...
11. ### Coming up with the critical density formula

Homework Statement "Imagine a galaxy with mass m at a distance r away from the center of a sphere, within which a total mass M reside. As viewed by an observer in the center, the galaxy appears to be receding according to the Hubble's law, v = H0r. To heuristically derive the critical density...
12. ### Critical density in our expanding universe

I am only a beginner in physics and have been reading popular books. I read that the average mass-energy density of the universe determines its shape such as having uniform positive curvature, uniform negative curvature, or zero curvature if it equals the critical density. However, if spacetime...
13. ### Calculating the critical density

Homework Statement I can't get how they calculate that. I'm not sure if there including H0 which I think is 70 or not but I've tried with and without and both don't work. I get (3 * 70^2)/(8pi*6.67 * 10^-11) which is way off. I also can't get the units that they arrive...
14. ### Critical density and total observable mass

Assuming a Hubble constant of 74.3 km/sec/Mpc, the critical density is about E-29 gm/cm3. To calculate observable matter based on 5% of this density (the other 95% is dark matter and dark energy), a volume has to be used. But which volume, the one with a 13.7 billion light year radius or the...
15. ### Critical density and observable mass

Please comment on my assumptions and results, this has confused me for some time. Thanks The following is an attempt to reconcile the critical density of the universe with the amount of observable matter as represented by number of stars. Assumptions for critical density: Hubble constant...
16. ### Critical density formula hides fundamental relationship?

Observations show that the current density of the Universe is close to the critical density: rho = 3 H_0^2 / 8 Pi G Assuming a spherical observable Universe of constant density, together with the relation c = H_0 * R, one can rearrange the above equation to give the relation: G * M / R...
17. ### What is the critical density of the Universe using hubble's constant?

Now I'm using p=3H^2/8piG and I have H=70 km/s/Mpc, what is the critical density of the universe? now I'm plugging in all the values and I can never ever get something close to the densities calculated in the theories (something like 1.06x10^-28 g/m^3) in fact I can never get anything...
18. ### Critical Density: What Is It and Where Does It Come From?

What exactly does the critical density mean? In articles they keep comparing the desity of the universe to the critical density and what the consequences are but nobody explains what exactly the critical density is and where it comes from.
19. ### What is meant by density in units of critical density?

what is meant by density, such as the baryonic matter density in cosmology, in "units of the critical density"? its mentioned in a question i need to do but i don't really know what it means.
20. ### Critical density in Early Times

What was the total mass energy density at the time of recombination? Has it has it been measured by WMAP? (Edit: I think it's 4.28x10-18kg/m3 from the # of photons/volume in a 2979K blackbody, x baryon:photon ratio x proton mass x (1+ dark:baryon ratio) divided by the 0.755 matter fraction at...
21. ### Calculating Critical Density Using FRW and Its Implications

Using the FRW: \left( \frac {\dot{a}} {a} \right)^2 = \frac {8 \pi G \rho} {3} - \frac {k c^2} {a^2} We define critical density by setting k = 0 and rearranging to get: \rho_c = \frac {3 H^2} {8 \pi G} Where: H = \left( \frac {\dot{a}} {a} \right) My question is does \rho include the...
22. ### Critical density of water - phase transitions

Hello people, I'm studying now phase transitions. I saw that the order parameter of water gas-liquid transition is the d[g]-d[c] or d[l]-d[c]. where d[g] is gas density d[l] is liquid density d[c] is the critical density Can someone please elaborate more about the critical density...
23. ### The new Hubble rate estimate of 74 means a new critical density.

Around 1998 we got the estimate of 71 from Wendy Freedman's Hubble Space Telescope team. That was called the "key project" of the HST program and was one of the main reasons for HST. So much depends on it that it is important to keep trying to improve the accuracy. That 71 has stood for 10...
24. ### Critical Density: H0=70km/s.MPC, Fate of Universe

hello, can anyone help me with this problem please: For H0=70 km/ S . MPC , what is the critical density (p0) for the universe in gm/cm^3? . if the average density of the universe is 5 * 10 ^ -28 gm/cm^3 , what is the fate of the universe. thanks ,
25. ### Calculating Critical Density and Cosmological Constant

I'm currently looking into the values for the 'critical density' and 'cosmological constant', I managed to calculate a figure for the critical density which was close to the generally accepted figure, with lambda I came up with an astronomically small number which I later realized after...
26. ### Critical Density: What is the Energy Density of the Universe?

Do we know what the critical density of the universe is? I always thought we did know, but apparently (according to friend) we don't...
27. ### Critical Density and Vander Waal's Equation

" Critical Density of a substance having molecular weight is 0.555 gm/cc and the critical pressure is 48 atm. Calculate the Vander Waals constants 'a' and 'b'. " Ans: a= 5.645 atm lit^2/mol^2 b= 0.066 lit/mol My dear friendS! it looks like to me that the data is incomplete here as...
28. ### What is the critical density of the universe?

The critical density of the universe is (3c2H02)/8piG which is the average energy density required for flatness. And in fact recent measurements imply that our universe is indeed flat and therefore has this energy density. In addition, the Hubble parameter H0 has been determined in...