<p>As far as I understood it, no one really knows. It is observable that the universe is expanding with increasing speed, therefore there has to be a form of energy driving it. We can meassure (or at least try) the amount of energy and find $\Omega_\lambda = 69.11\%$ (see <a href="https://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters" rel="nofollow">$\lambda$CDM-Model</a>)</p>

<p>To your questions:</p>

<p>1: I wouldn't imagine it as Big Bang momentum, for it is also changing. After the Big Bang, there was (or at least had to be, to make our comological model work) a period of inflation, meaning rapid expansion, which then slowed down again. I can't think of an explanation which includes some kind of finite momentum.</p>

<p>2: On small scales (speaking of galaxies), where densities are high, expansion doesn't act. Gravity is working against it.</p>

<p>3: It is responsible for the expansion. How exactly is not understood. (Little personal comment form my side. If a physicist calls something 'dark', it mostly likely means it is not understood)</p>

CC BY-SA 3.0
2016-02-10T22:10:29.480Z
2: Gravity _is_ slowing down the expansion of the Universe as a whole, and was dominating until 3.5 Gyr ago. Only "recently" has dark energy been dominating.
pela
CC BY-SA 3.0
2016-02-11T00:30:48.503Z
3: The *how* dark energy is responsible for acceleration (which is what I gather you meant by 'expansion') is well-understood: it follows rather trivially and directly from the Einstein field equation. What's not understood is the reasons/origins for the sign and magnitude of the dark energy.
Stan Liou
CC BY-SA 3.0
2016-02-11T03:15:15.523Z
I think this question has been asked before. I'll comment here what I commented before. If I placed a bet on this, I would wager that dark energy is the result of a pressure caused by the energy in empty space. The real question should be " Why don't we find a lot more energy?"
Jack R. Woods
CC BY-SA 3.0
2016-02-11T19:46:03.957Z
@pela Thank you. I just learnt a new unit - Gyr! Sounds like dark energy is releasing pent-up stress from Gravity's huddle up.
signsgeek
CC BY-SA 3.0
2016-02-11T19:51:46.173Z
@StanLiou Thanks for answering my unstated question as well! So is it not currently known how the dark energy is formed? Also thanks for bringing up Einstein field equation. As a physics enthusiast I will dig deeper into it. Grateful for the leads!
signsgeek
CC BY-SA 3.0
2016-02-11T19:56:52.750Z
@JackR.Woods Thank you for your comment. Is your question based on a set of equations or theory? If yes I'd like to read up on it and get some background. Also is it possible for the Universe to reach a point where the energy dominates the mass at a much larger ratio than what it is currently? Interesting since the earth kind of reflects a similar model where the water mass (at least on the surface) is more than the land mass.
signsgeek
CC BY-SA 3.0
2016-02-11T19:59:21.623Z
Just wanted to add that original answers as well as all your posts contribute to answering my questions. I don't see a way to indicate that in the current tool, so I'll mark the original answer as the answer. Thanks
signsgeek
CC BY-SA 3.0
2016-02-12T23:51:21.840Z
@signsgeek watch this www.youtube.com/watch?v=GgKGPyFL-TM . I'm sure you will find it fascinating (and the math really isn't that bad). As regards to your question in the comment: "I'm sure you are aware that energy already accounts for 70% of the mass of the known universe". If Susskind and others are correct then the answer is "Yes. As space expands, the amount of energy increases. In addition the expansion of space should approach an exponentially accelerating expansion."
Jack R. Woods
CC BY-SA 3.0
2016-02-15T22:48:31.883Z
@JackR.Woods thanks for the link! I'll dig through the gems that the Prof. provides, will take a few re-runs.
signsgeek
CC BY-SA 3.0
2016-02-10T21:26:14.537Z
2016-02-10T21:26:14.537Z
rtime
<ol>
<li>Is it just the momentum from the Big Bang? If yes, how can it account for the acceleration of the expansion?</li>
<li>Why isn't the collective gravitational force slowing it down?</li>
<li>What part does dark energy play in the expansion?</li>
</ol>

<universe><expansion>
What is driving the expansion of the Universe?
signsgeek
# What is driving the expansion of the Universe?

(Asked by: username_0 on 2016-02-10)

1. Is it just the momentum from the Big Bang? If yes, how can it account for the acceleration of the expansion?
2. Why isn't the collective gravitational force slowing it down?
3. What part does dark energy play in the expansion?

**Tags:** universe, expansion

---

## 1 answers

### Answer by username_1 on 2016-02-10. Score: 3 | Accepted answer

As far as I understood it, no one really knows. It is observable that the universe is expanding with increasing speed, therefore there has to be a form of energy driving it. We can meassure (or at least try) the amount of energy and find $\Omega\_\lambda = 69.11\%$ (see [$\lambda$CDM-Model](https://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters))

To your questions:

1: I wouldn't imagine it as Big Bang momentum, for it is also changing. After the Big Bang, there was (or at least had to be, to make our comological model work) a period of inflation, meaning rapid expansion, which then slowed down again. I can't think of an explanation which includes some kind of finite momentum.

2: On small scales (speaking of galaxies), where densities are high, expansion doesn't act. Gravity is working against it.

3: It is responsible for the expansion. How exactly is not understood. (Little personal comment form my side. If a physicist calls something 'dark', it mostly likely means it is not understood)

> **Comments:**
> **username_2**: 2: Gravity _is_ slowing down the expansion of the Universe as a whole, and was dominating until 3.5 Gyr ago. Only "recently" has dark energy been dominating.  
> **username_3**: 3: The *how* dark energy is responsible for acceleration (which is what I gather you meant by 'expansion') is well-understood: it follows rather trivially and directly from the Einstein field equation. What's not understood is the reasons/origins for the sign and magnitude of the dark energy.  
> **username_4**: I think this question has been asked before. I'll comment here what I commented before. If I placed a bet on this, I would wager that dark energy is the result of a pressure caused by the energy in empty space. The real question should be " Why don't we find a lot more energy?"  
> **username_0**: @pela Thank you. I just learnt a new unit - Gyr! Sounds like dark energy is releasing pent-up stress from Gravity's huddle up.  
> **username_0**: @StanLiou Thanks for answering my unstated question as well! So is it not currently known how the dark energy is formed? Also thanks for bringing up Einstein field equation. As a physics enthusiast I will dig deeper into it. Grateful for the leads!  
> **username_0**: @JackR.Woods Thank you for your comment. Is your question based on a set of equations or theory? If yes I'd like to read up on it and get some background. Also is it possible for the Universe to reach a point where the energy dominates the mass at a much larger ratio than what it is currently? Interesting since the earth kind of reflects a similar model where the water mass (at least on the surface) is more than the land mass.  
> **username_0**: Just wanted to add that original answers as well as all your posts contribute to answering my questions. I don't see a way to indicate that in the current tool, so I'll mark the original answer as the answer. Thanks  
> **username_4**: @signsgeek watch this www.youtube.com/watch?v=GgKGPyFL-TM . I'm sure you will find it fascinating (and the math really isn't that bad). As regards to your question in the comment: "I'm sure you are aware that energy already accounts for 70% of the mass of the known universe". If Susskind and others are correct then the answer is "Yes. As space expands, the amount of energy increases. In addition the expansion of space should approach an exponentially accelerating expansion."  
> **username_0**: @JackR.Woods thanks for the link! I'll dig through the gems that the Prof. provides, will take a few re-runs.  

---

