What is Friedmann equation: Definition and 39 Discussions
The Friedmann equations are a set of equations in physical cosmology that govern the expansion of space in homogeneous and isotropic models of the universe within the context of general relativity. They were first derived by Alexander Friedmann in 1922 from Einstein's field equations of gravitation for the Friedmann–Lemaître–Robertson–Walker metric and a perfect fluid with a given mass density
ρ
{\displaystyle \rho }
and pressure
p
{\displaystyle p}
. The equations for negative spatial curvature were given by Friedmann in 1924.
"Tsinghua students, true to form, protesting lockdowns with the Friedmann Equation: the basic reality of the universe is constant, eternal expansion, or put another way, opening up."
[Moderator's note: chinese twitter link removed.]
If it is known that such an equation exists, I would much appreciate seeing a link to a reference. If it is known that such an equation does not exist, I would much appreciate seeing a reference with an explanation of implications regarding the compatibility of general relativity with a...
I was shown that adot^2/a^2 = c/a^3, adot = c / √(a), then da/dt = c / √(a) . Then I was told that I have to integrate this, but I don't understand where to go from there or how this will show me that the scale factor grows as t^(2/3).
Does the Friedmann vacuum equation have a linear solution rather than an exponential one?
Using natural units one can write Friedmann's equation for the vacuum as
$$
\begin{eqnarray*}
\left(\frac{\dot a}{a}\right)^2 &=& \frac{8\pi G}{3}\rho_{vac}\\\tag{1}
&=& L^2 \left(\frac{\rho_0}{L^4}\right)...
One simple question whether the k in the friedmann equation
H(t)2=∑8πGε(t)/3c2-k/a2
is something related to curvature or is simply constt.??
If related to curvature whether it is 1/R where R is the radius of the 3-sphere.??
Homework Statement
Assume that the universe today is flat with both matter and a cosmological constant, the latter with energy density which remains constant with time. Integrate Eq. (1.2) to find the present age of the universe. That is, rewrite Eq. (1.2) as:
dt = H0-1 da / a [ΩΛ + (1 - ΩΛ )...
Greg Bernhardt submitted a new PF Insights post
This article is part of our student writer series. The writer Arman777, is an undergraduate physics student at METU
A Journey Into the Cosmos - FLRW Metric and The Friedmann Equation
Continue reading the Original PF Insights Post.
Friedmann Equation without the cosmological constant can be written as;
$$H^2=\frac {8πρG} {3}-\frac {1} {a^2(t)}$$
for ##Ω>1## (which in this case we get a positive curvature universe), and for a matter dominant universe;
$$ρ=\frac {1} {a^3(t)}$$
so in the simplest form the Friedmann equation...
while deriving the friedmann equation using Newtonian Mechanics the 2nd term of the r.h.s is coming to be 2^U/(r^2*a^2) where U is a constt,but it is replaced by -kc^2/(r^2*a^2)?
Homework Statement
Use Friedmann equations to show that if ##\dot{a} > 0##, ##k<0## and ##\rho>0## then there exists a ##t*## in the past where ##a(t*)=0##
Homework Equations
[/B]
Friedmann :
##(\frac{\dot{a}}{a})^2=\frac{8\pi G}{3}\rho-\frac{k}{a^2}##The Attempt at a Solution
[/B]...
Friedmann's Eq can be viewed here https://ned.ipac.caltech.edu/level5/March08/Frieman/Equations/paper1x.gif
What I don't get is that all the texts/analyses of Friedmann's equation say that if the right hand side is negative it means that the universe will expand reach a critical point and then...
It has occurred to me that the Friedmann equation
allows for a solution
H = H0 = 0 .
This seems to say that independently of the value of the scale factor a, and the various Ωs, a static universe is possible. I am guessing that this solution is spurious and is a side effect of the...
Hi all
I want to ask a question about NFE(Newtonian Friedmann Equation).I know that NFE is not usefull to describe universe.But we can have a general idea about universe to use that formula.
I know that the only spatial coordinate system is CMB referance frame and NFE is derived from...
In friedmann equation we start to make a model
kinetic energy-potantial energy=U
kinetic energy depends observer and referance frame so what's the referance frame and observer in this calculation.
Thanks
Homework Statement
For the equation H^2 = \frac{8 \pi G \rho_m}{3} + \frac{H}{r_c} how do I find the value of H for scale factor a \rightarrow \infty , and show that H acts as though dominated by \Lambda (cosmological constant) ?
Homework Equations
\rho_m \propto \frac{1}{a^3}
H > 0...
Definition/Summary
The Friedmann equation is a dynamical equation that describes the expansion of the universe.
Equations
H^2 = \left( \frac{\dot{a}}{a}\right)^2 = \frac{8\pi G}{3} \rho - \frac{kc^2}{a^2}
Extended explanation
The Friedmann equation is derived from the 0-0...
The Hubble's Law v=H0D
LHS of The Friedmann equation H2/H02
I am a bit confused with the following:
1: The Hubble constant H0 is the current universe velocity/distance ratio, it is basically a constant, so galaxies in the universe with a further distance, move away from each other at a...
Homework Statement
Consider a universe with Cold Matter, Radiation, and Dark Energy satisfying an equation of
state p = w\rho.
Variables: p is the pressure of the universe, w is a constant, and \rho is the density of the universe.
a) Show that the Friedmann equation can be rewritten as...
Homework Statement
To relate E to the total energy of the expanding sphere, we need to integrate over
the sphere to determine its total energy. These integrals are most easily carried out
by dividing the sphere into shells of radius r, and thickness dr, so that each shell
has a volume dV...
I am having a trouble finding relationship between 'a' and 't' by integrating friedmann equation in a multi-component universe.
It would be very helpful if you can help me with just
matter-curvature only universe and matter-lambda only universe.
The two integrals looks like following...
Homework Statement
the first friedmann equation is:
(\frac{\dot{a}}{a})^2=\frac{8\pi G\rho}{3}-\frac{kc^2}{a^2}+\frac{\Lambda}{3}
In the case of a closed Universe (k > 0) containing only non-relativistic matter and no cosmological constant, write the Friedmann equation in terms of, H(a), H0...
Looking at the Friedmann equation
H^2=\left[\frac{\dot{a}}{a}\right]^2=\frac{8\pi G\rho}{3}-\frac{kc^2}{a^2}
and considering positive curvature, then for the limit where the second term dominates, we're left with
\left[\frac{\dot{a}}{a}\right]^2=-\frac{kc^2}{a^2}
This implies a complex...
(i) Obtain the scale factor a(t) and redshift z when the energy density of matter and radiation were equal.
(ii) Next use the a(t) relation for a matter-only universe to estimate the time of matter-radiation equality.
(iii) Repeat (ii) but using the a(t) relation for a radiation-only universe...
I'm just beginning to look into loop quantum cosmology, and I've recently run into some fundamental confusion concerning how exactly the Friedmann equation fits in. Is it just that the application of loop quantum gravity yields a modified/effective Friedmann equation, or is it the other way...
Are you confortable with the demonstration in page 19 of Introduction to Modern Cosmology - A. Liddle ? My problem is in particular eq. (3.4) when he writes the potential energy of a test particle. I think there is a factor of 2 missing in the denominator.
I'm a little confused about the density \rho in the equation:
H^2 = \left(\frac{\dot{a}}{a}\right)^2 = \frac{8 \pi G}{3}\rho - \frac{kc^2}{a^2}
Measuring \rho at a single instant in time seems easy. But \rho changes with time. The time dependence of \rho is given as...
Homework Statement
The universe is expanding as shown by a Hubble constant H of 75 Km s^-1 Mpc^-1, but a constant density of matter of 10^-26 kg m^-3 is maintained by the continuous creation of hydrogen atoms as the universe expands. Find the rate of creation of hydrogen atoms per unit...
Hey! I'd like to see an easy way to arrive to Friedmann equation by using Newton's Mechanics. I've seen many ways but they skip many steps so I don't understand the whole.
THANKS!
Homework Statement
Solve for the age of the universe as a function of scale factor in a cosmology with \Omega_{M}=0.3, \Omega_{\Lambda}=0.7.Homework Equations
The Attempt at a Solution
(\frac{\dot{a}}{a})^2=H_0^2(0.3a^{-3}+0.7)
\frac{da}{a\sqrt{0.3a^{-3}+0.7}}=H_0dt
At this point, Maple seems...
Anybody like to help a physics student who did their BSc six years ago and has forgotten all their maths!
Its a problem from Andrew Liddle's Introduction to cosmology 5.5
The Friedmann eqn is
(a'/a)2 = 8TTG/3 P - k/a2
Consider the case k>o, with the universe containing matter only, so...
Homework Statement
Show that a lambda-dominated euclidean universe entails an exponential expansion.
Homework Equations
Equation of state is P=-E (E=epsilon)
There is no curvature (i.e. a flat universe)
The Friedmann equation is then
((a(dot)/a)^2)=((8piG)/3c^2)E + lambda/3...
Homework Statement
By substituting in
\left( \frac{\dot{a}}{a_0} \right)^2 = H^2_0 \left(\Omega_0 \frac{a_0}{a} + 1 - \Omega_0 \right)
show that the parametric open solution given by
a(\psi)=a_0 \frac{\Omega_0}{2(1-\Omega_0)}(\cosh{\psi} - 1)
and
t(\psi)=\frac{1}{2H_0} \frac{\Omega_0}{(1 -...
I'm trying to derive Friedmann's equation for cosmology using Newtonian physics. I've got the force equation F=ma for the case without a cosmological constant. But now I'm trying to incorporate the cosmological constant into this force equation.
But I'm having trouble seeing how the...
Hi does anyone know a website or paper where the friedmann equation is derived from the robertson walker metric?
it should have the calculation of atleast a few of the christoffel symbols etc
thanks
blumfeld
Got stuck in this question, the question asked to use friedmann equation to show that the age of an empty universe (average density = 0) is
τ=1/H₀
where H₀ is the present day value of Hubble's Constant.
Well, i got the friedmann equation by put the value ρ(average)=0 but the trouble is...
arXiv:astro-ph/0503715 v2 31 Mar 2005
Late time failure of Friedmann equation
Alessio Notari*
Physics Department, McGill University, 3600 University Road, Montr´eal, QC, H3A 2T8, Canada
(Dated: March 31, 2005)
It is widely believed that the assumption of homogeneity is a good zeroth...