<p>They are just fractions. You can think of them as percentages of the amount of mass/energy needed to make spacetime flat.</p>
<p>The large-scale shape of spacetime could be curved.  That curvature could be positive or negative. &quot;Positive&quot; would mean that parallel beams of light would eventually get closer together.  &quot;Negative&quot; means that parallel beams of light would diverge. The amount of curvature depends on the amount and density of mass/energy (&quot;stuff&quot;) in the universe relative to its expansion.  So I can talk about the amount of &quot;stuff&quot; needed to make the shape of the universe &quot;flat&quot; with zero curvature (so parallel beams of light stay parallel)</p>
<p>Ordinary matter is a percentage of this amount, but a small percentage of the stuff needed to make spacetime flat:  about 5%.  Our models suggest that there is other matter, &quot;dark matter&quot;, that has mass, but isn't made of normal atoms. It is about 26% of the stuff needed to make the universe flat. That's a total of about 31%.  Light is a form of energy, and there are other particles like neutrinos.  These also contribute to the total amount of stuff in the universe, but it turns out that these are small contributions, less than one percent. However &quot;dark energy&quot; (which may not be energy at all, but just a property of spacetime) fills the universe and contributes 69% of the stuff needed to make space-time flat.  You'll notice that 31+69=100%.  This is the observation that, on the large scale, space-time appears to be flat. The total amount of stuff in the universe is 100% of what is needed to make the universe flat.</p>

CC BY-SA 4.0
2024-12-21T10:38:45.213Z
I'm not sure to understand the problem of "light curved by mass" in your explanation : if there is one mass the light is curved around it, ok. But if there are two mass (or more, regularly placed) around a ray of light it will not be curved anymore no ? The space is full of matter so the "average" deviation of light is null so it's as if there was nothing no ?
Fabien
CC BY-SA 4.0
2024-12-21T13:29:15.163Z
Remember that the universe is expanding.  If there was no matter at all in the universe, there would be nothing to slow the expansion, and light paths would diverge.  If there was a lot of "stuff" then it would curve space time and reverse expansion: and light paths would converge (in fact this leads to a "big cruch") But it is possible that there is just the right amount of stuff to exactly balance the initial expansion.
James K
CC BY-SA 4.0
2024-12-21T13:30:14.537Z
I mention the idea of beams of light, since this can be understood without needing a viewpoint from "outside" the universe.  You can understand what "curved spacetime" is without it being embedded in a higher dimension space. It can be understood intrinsically
James K
CC BY-SA 4.0
2024-12-21T15:18:55.583Z
I think if there was no matter at all in the universe, we could not say that the universe is expanding nor crushing or flat, it's the matter in the universe who "expand", i.e. get away "from each other", like in an explosion. And I don't understand what you mean by "light paths would converge"... The light always goes at the speed of c and straight except when it's deviated by mass, how can it "all converge" ?
Fabien
CC BY-SA 4.0
2024-12-21T23:15:01.450Z
It converges because space time is curved.  Look just forget I said anything about light paths.  It is a minor detail. The point is that the various Omegas are percentages of the amount of stuff needed to eventually stop expansion.
James K
CC BY-SA 4.0
2024-12-21T09:33:14.287Z
2024-12-21T13:34:44.757Z
2024-12-21T13:34:44.757Z
James K
<p><a href="https://en.wikipedia.org/wiki/Friedmann_equations" rel="nofollow noreferrer">See Wiki on Friedmann equations</a>. To quote:</p>
<blockquote>
<p>The density parameter <span class="math-container">$\Omega$</span> is defined as the ratio of the actual (or observed) density <span class="math-container">$\rho$</span> to the critical density <span class="math-container">$\rho_c$</span> of the Friedmann universe</p>
</blockquote>
<p>From there, the subscript indicates which component of the universe is being referred to (<span class="math-container">$\Omega_m$</span> = the density parameter for mass, for example).</p>
<p>Beyond that, it's just notation; you could use another symbol for <span class="math-container">$\Omega$</span>, but you'd have to explain your notation first because it's standard notation in cosmology (and you'd still confuse people who'd wonder why you don't just use the standard notation).</p>

CC BY-SA 4.0
2024-12-21T15:10:44.370Z
Yes, I know I have to study Friedmann equations, but it's quite a complicated thing... I'm not sure I can afford it, that's why I tried to ask here but I have the feeling it won't be possible to understand "without mathematics"... I find a bit sad (and even suspicious) that something "physical" cannot be explained without "mathematics".
Fabien
CC BY-SA 4.0
2024-12-21T14:04:13.653Z
2024-12-21T14:04:13.653Z
Allure
<p>Is there a &quot;simple&quot; way to explain cosmological parameters ΩM, ΩΛ and other ?</p>
<p>I know it must rely on gravitation and density of matter of course, but I don't get all those &quot;Ω&quot;...</p>
<p>My level in math is not so high  (around &quot;Associate’s Degree&quot; level of French studies) so this is why I ask if there is a &quot;simple enough&quot; way to still understand the cosmological parameters' notion.</p>

<universe><expansion>
Is there a "simple" way to explain cosmological parameters?
Fabien
# Is there a "simple" way to explain cosmological parameters?

(Asked by: username_0 on 2024-12-21)

Is there a "simple" way to explain cosmological parameters ΩM, ΩΛ and other ?

I know it must rely on gravitation and density of matter of course, but I don't get all those "Ω"...

My level in math is not so high (around "Associate’s Degree" level of French studies) so this is why I ask if there is a "simple enough" way to still understand the cosmological parameters' notion.

**Tags:** universe, expansion


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## 2 answers

### Answer by username_1 on 2024-12-21. Score: 1 | Accepted answer

They are just fractions. You can think of them as percentages of the amount of mass/energy needed to make spacetime flat.

The large-scale shape of spacetime could be curved. That curvature could be positive or negative. "Positive" would mean that parallel beams of light would eventually get closer together. "Negative" means that parallel beams of light would diverge. The amount of curvature depends on the amount and density of mass/energy ("stuff") in the universe relative to its expansion. So I can talk about the amount of "stuff" needed to make the shape of the universe "flat" with zero curvature (so parallel beams of light stay parallel)

Ordinary matter is a percentage of this amount, but a small percentage of the stuff needed to make spacetime flat: about 5%. Our models suggest that there is other matter, "dark matter", that has mass, but isn't made of normal atoms. It is about 26% of the stuff needed to make the universe flat. That's a total of about 31%. Light is a form of energy, and there are other particles like neutrinos. These also contribute to the total amount of stuff in the universe, but it turns out that these are small contributions, less than one percent. However "dark energy" (which may not be energy at all, but just a property of spacetime) fills the universe and contributes 69% of the stuff needed to make space-time flat. You'll notice that 31+69=100%. This is the observation that, on the large scale, space-time appears to be flat. The total amount of stuff in the universe is 100% of what is needed to make the universe flat.

> **Comments:**
> **username_0**: I'm not sure to understand the problem of "light curved by mass" in your explanation : if there is one mass the light is curved around it, ok. But if there are two mass (or more, regularly placed) around a ray of light it will not be curved anymore no ? The space is full of matter so the "average" deviation of light is null so it's as if there was nothing no ?  
> **username_1**: Remember that the universe is expanding.  If there was no matter at all in the universe, there would be nothing to slow the expansion, and light paths would diverge.  If there was a lot of "stuff" then it would curve space time and reverse expansion: and light paths would converge (in fact this leads to a "big cruch") But it is possible that there is just the right amount of stuff to exactly balance the initial expansion.  
> **username_1**: I mention the idea of beams of light, since this can be understood without needing a viewpoint from "outside" the universe.  You can understand what "curved spacetime" is without it being embedded in a higher dimension space. It can be understood intrinsically  
> **username_0**: I think if there was no matter at all in the universe, we could not say that the universe is expanding nor crushing or flat, it's the matter in the universe who "expand", i.e. get away "from each other", like in an explosion. And I don't understand what you mean by "light paths would converge"... The light always goes at the speed of c and straight except when it's deviated by mass, how can it "all converge" ?  
> **username_1**: It converges because space time is curved.  Look just forget I said anything about light paths.  It is a minor detail. The point is that the various Omegas are percentages of the amount of stuff needed to eventually stop expansion.  

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### Answer by username_2 on 2024-12-21. Score: 1

[See Wiki on Friedmann equations](https://en.wikipedia.org/wiki/Friedmann_equations). To quote:

> The density parameter $\Omega$ is defined as the ratio of the actual (or observed) density $\rho$ to the critical density $\rho\_c$ of the Friedmann universe

From there, the subscript indicates which component of the universe is being referred to ($\Omega\_m$ = the density parameter for mass, for example).

Beyond that, it's just notation; you could use another symbol for $\Omega$, but you'd have to explain your notation first because it's standard notation in cosmology (and you'd still confuse people who'd wonder why you don't just use the standard notation).

> **Comments:**
> **username_0**: Yes, I know I have to study Friedmann equations, but it's quite a complicated thing... I'm not sure I can afford it, that's why I tried to ask here but I have the feeling it won't be possible to understand "without mathematics"... I find a bit sad (and even suspicious) that something "physical" cannot be explained without "mathematics".  

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