# Omega being less than one in a flat Universe?

• TheTraceur
In summary, the conversation discusses the Friedmann equation and its implications for a flat universe. It is explained that when considering a flat universe, the density parameter Ω0 must be slightly smaller than 1 due to the presence of dark energy. The conversation also delves into the role of the Hubble parameter and the critical density in understanding the curvature of the universe. It is ultimately concluded that if Ωtotal is not equal to 1, then the curvature of the universe is non-zero.
TheTraceur
Hi everyone! I'm not very good at math as I'm in high school, and so probably did something wrong in this, but I did some calculations and am now wondering if a flat Universe requires Ω0 ≠ 1?

I started with the Friedmann equation,

If you divide by 3 and take the inverse of both sides, you get,

Since,

Where ρc is the critical density, we can re-write the equation in terms of Omega (I also multiplied both sides by c2),

Here it to me looks a bit odd, because here Omega being one does not correspond to a Universe with infinite radius of curvature, since you still have to cosmological constant, which is positive. Here it seems that if you want to assume that k = 0, and so have a flat Universe, you will need to have (since if R2/k, where k = 0, is 'infinite', then the other side must also be 'infinite'):

We then get:

And since the cosmological constant has been discovered to be positive, this would seem to mean that for a flat Universe (k = 0), Ω0 is not exactly equal to one, but in fact a tiny bit smaller. I'm pretty sure this is false, but I don't know where I've missed the fault in the derivation. May it be that the cosmological constant is assumed to be zero in the definition of critical density? Because if you don't assume a zero value for the cosmological constant in the definition of critical density, the cosmological constant dissapears in the denominator, and so Omega being one corresponds to a flat Universe.

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Welcome to PF, TheTraceur!

TheTraceur said:
Hi everyone! I'm not very good at math as I'm in high school, and so probably did something wrong in this, but I did some calculations and am now wondering if a flat Universe requires Ω0 ≠ 1?

I started with the Friedmann equation,

If you divide by 3 and take the inverse of both sides, you get,

Since,

Where ρc is the critical density, we can re-write the equation in terms of Omega (I also multiplied both sides by c2),

Here it to me looks a bit odd, because here Omega being one does not correspond to a Universe with infinite radius of curvature, since you still have to cosmological constant, which is positive. Here it seems that if you want to assume that k = 0, and so have a flat Universe, you will need to have (since if R2/k, where k = 0, is 'infinite', then the other side must also be 'infinite'):

The thing you're missing here is that the ρ in your equation is the density of matter, ρm. So when you compute Ω0 = ρmcr, you're only computing Ωm, the density parameter for matter. This is less than 1: in the standard concordance model, we have Ωm = 0.27 and Ωde = 0.73, where Ωde (also called ##\Omega_\Lambda##) is the density parameter for dark energy. As you know, observations show that the total matter density is far less than the critical density, and it is the dark energy that makes up the difference, bringing the universe very close to, if not exactly, critical (flat).

So, how can your equation be reworked to include Ωde? Well, for the simplest type of dark energy, which is just represented by a cosmological constant term in the Friedmann equations, it turns out that the dark energy density ρde is constant with time, and is related to the cosmological constant by: ##\Lambda = 8\pi G \rho_{\textrm{de}}##

Substituting this into your equation above, we get (note, I'll just take c = 1 for convenience):$$H^2 = \frac{8\pi G}{3}\rho_m + \frac{8\pi G}{3}\rho_{\textrm{de}} - \frac{k}{R^2}$$Divide both sides of the equation by H2:$$1 = \frac{8 \pi G}{3H^2}\rho_m + \frac{8 \pi G}{3H^2}\rho_{\textrm{de}} - \frac{k}{R^2 H^2}$$Now, the important thing to understand in this equation is that a lot of these things are functions of time, i.e. their values change with cosmic time, t (the time since the Big Bang, according to observers who are co-moving with the expansion). The Hubble parameter, H(t) is a function of time. So is the matter density ρm(t). So is the scale factor R(t). I am assuming that you are using R to denote the scale factor. I usually use 'a', but that's okay. So, let's evaluate this equation NOW i.e. at t = t0, where t0 is the present age of the universe (some 13.7 billion years). It turns out that the present value of the Hubble parameter, H(t0), is just equal to the Hubble constant, H0. The present value of the scale factor, R(t0) = 1. I'll call the present value of the matter density ρm,0. So at t = t0 the equation becomes:$$1 = \frac{8 \pi G}{3H_0^2}\rho_{m,0} + \frac{8 \pi G}{3H_0^2}\rho_{\textrm{de}} - \frac{k}{H_0^2}$$ By now, you probably recognize that the factors in front of the ρ's are just equal to the reciprocal of the critical density: ##\rho_{\textrm{cr}} = 3H_0^2 / (8\pi G)##. So the equation becomes:$$1 =\frac{\rho_{m,0}}{\rho_{\textrm{cr}}} + \frac{\rho_{\textrm{de}}}{\rho_{\textrm{cr}}} - \frac{k}{H_0^2}$$By the definition of the Ω's, this is just equal to:$$1 = \Omega_m + \Omega_{\textrm{de}} - \frac{k}{H_0^2}$$OR$$\frac{k}{H_0^2} = (\Omega_m + \Omega_{\textrm{de}}) - 1$$$$\frac{k}{H_0^2} = \Omega_{\textrm{total}} - 1$$So if Ωtotal (including contributions from both matter and dark energy) differs from 1, then the curvature k is non-zero. Some people even go so far as to define a "curvature" density parameter Ωk = -k/H02, so that 1 = Ωm + Ωde + Ωk.

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## 1. What does it mean for Omega to be less than one in a flat Universe?

In cosmology, Omega (Ω) is a measure of the density of matter and energy in the Universe, compared to the critical density needed for the Universe to eventually stop expanding. In a flat Universe, Omega being less than one means that the amount of matter and energy present is not enough to halt the expansion of the Universe, and it will continue to expand forever.

## 2. How is Omega calculated in a flat Universe?

To calculate Omega in a flat Universe, we use the following equation: Ω = (density of matter + density of dark energy) / critical density. The critical density is determined by the amount of mass and energy needed for the Universe to eventually stop expanding. If the calculated value of Omega is less than one, it means that the Universe will continue to expand indefinitely.

## 3. What are the implications of Omega being less than one?

If Omega is less than one in a flat Universe, it means that the expansion of the Universe will continue forever. This has significant implications for the ultimate fate of the Universe, as it suggests that the expansion will not slow down or reverse, but will continue to accelerate. This also has implications for the distribution of matter and energy in the Universe, as well as the potential existence of other universes beyond our own.

## 4. How do we know that Omega is less than one in a flat Universe?

Scientists have measured the density of matter and dark energy in the Universe using various techniques, such as analyzing the cosmic microwave background radiation and studying the movements of galaxies. These measurements have consistently shown that the value of Omega is less than one, indicating that the Universe is flat and will continue to expand forever.

## 5. Could Omega ever change in a flat Universe?

While Omega can change over time, it is unlikely to change significantly in a flat Universe. This is because the amount of matter and energy in the Universe is relatively constant, and the expansion of the Universe is not expected to slow down or reverse. However, there are some theories that suggest Omega could change if other universes have a significant impact on our own, but this is still a subject of ongoing research and debate in the scientific community.

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