# Why is the universe expanding if only gravity is present between planets in space?

it has been proved by measurements that other galaxies are going far away with an acceleration which implies there is an other force between planets which is dominant than gravity. is there some anti-gravity type force there? if yes, then why it is not acting between earth and moon or sun and planets?

Note by Kautsya Kanu
7 years, 4 months ago

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1. Galaxies can be moving away from us without any other force besides gravity. Gravity would cause that expansion to slow down, but galaxies can have a velocity from the initial expansion of the universe without resorting to gravity or an additional force to provide that motion.

2. The observed acceleration of the galaxies does require a "force". We call the stuff in the universe responsible for this acceleration dark energy.

3. Who said dark energy is not acting on our solar system? What you have to realize is that the solar system is gravitationally bound and dark energy doesn't provide enough of a force to kick the planets out of the solar system. It does, however, have a slight effect on the orbits of the planets. This effect is exceedingly small. You can read here for the calculation of the size of the effect. In summary, nothing is going wrong with your intuition! Just remember that the magnitude of the effect is important too for the dynamics.

Staff - 7 years, 4 months ago

A static universe cannot be stable with just the Newton's gravitational force involved. The universe is expanding because the space itself is expanding. It is not a question of dark matter/dark energy. Even ordinary matter can cause expansion. Dark energy only accelerates the expansion. The general solution of Einstein's equations in General Relativity allow for existence of expanding/contracting universes. In fact, Newton way back in 17th century realized that the universe cannot be stable yet static at the same time. So he postulated an infinite universe. But now we know that universe can be finite and stable but not static. This static assumption has to be relaxed.

- 7 years, 4 months ago

How can space be finite?

- 7 years, 4 months ago

A person living on a sphere lives in finite space--i.e. finite surface area of his space. Similarly we in 3-dimensions can think of living in a higher dimensional space with finite volume.(3-D Volume)

- 7 years, 4 months ago

http://www.esa.int/OurActivities/SpaceScience/IstheUniverse_finite

- 7 years, 4 months ago

Dark matter/Dark energy. Its still pretty unknown, but first of, there is left over momentum from the Big Bang, which is insufficient to rip systems apart, but it pushes them along. Honestly though, you raise a good point. The reason why I (I repeat, just me, don't take my word for it) think that this happens is because the Dark force is causing space-time to expand, so the distance between objects is becoming greater. Here's the thing though, this force is not enough to spread counteract the gravitational force of galaxies, solar systems, stars and worlds, which are strong enough in a local area so that they stay together. Like a couple of balls with strings connecting them on top of a wave. They move away, but the ones with strings remain together because the force is not enough to separate them. Now, as gravity weakens with distance, this means that inter-galactic gravitation is not strong enough to hold them together (so basically no 'strings' pulling them together), but it is far stronger in star clusters e.g. the Milky Way.I hope that answers your question, though if it doesn't I'm sure someone smarter will come along soon enough and set you straight :)

- 7 years, 4 months ago

Yes you are right Ali

- 7 years, 4 months ago

Ya u r right that universe is expanding but at the same rate we are also expanding.............thats why we doesnt feel it.............and it is surely dark energy........

- 7 years, 4 months ago

A static universe cannot be stable with just the Newton's gravitational force involved. The universe is expanding because the space itself is expanding.>>> That's already say Balaji Dodda, india>>>>

- 7 years, 4 months ago

big bang happened due to a large repelling force(much much larger than gravity).....things formed in course of time and started drifting apart due to higher force. till universe's age was 7 billion, gravity tried to minimise the acceleration of expansion of universe. but after 7 billion years, unknowingly the universe expanded with an increasing, not decreasing, acceleration. it is caused due to dark energy accumulated b/w the galaxies. Since the dark energy exist b/w galaxies, and not inside the galaxy, it doesnot affect the distance b/w sun, moon, earth etc...
hope it helps...:D

- 7 years, 4 months ago

you are also right Rohan

- 7 years, 4 months ago

Not only gravity between planets is acting but also some unknown type of energy known as "Dark Energy" is also acting which is responsible for "expansion of universe" and as we all know that the Gravitational force is one of the weakest fundamental force in nature .Therefore it has not that much power to go against that dark energy.But no one knows how this energy was produced or how it can be overcomed .

- 7 years, 4 months ago

the ans of other questions will also be given by me but after dome time.O.K.

- 7 years, 4 months ago

sorry its some instead of dome

- 7 years, 4 months ago

its very complicated to answer such questions because the words which i use in my explanation are most often conveying the meaning which it is not intended to

- 7 years, 4 months ago

Nowdays people assume that the Universe is Holographic... a mere projection of the human brain. Sounds absurd.. but then everything does till its proven.

- 7 years, 4 months ago

The usual use of the word holography in gravitational physics refers to the idea that all the degrees of freedom we naively think of in a region of spacetime with 3 spatial dimensions + 1 time dimension can be encoded on the boundary of that region. It doesn't refer to the idea that the universe is the projection of someone's brain. Now, there are speculative ideas along those lines that are used to tell us something about cosmology. These are called Boltzmann's brains, and I highly encourage you to explore the web on Boltzmann's brains. It's an amusing story.

Staff - 7 years, 4 months ago

Yes Sudarshan some scientists think in this direction also.

- 7 years, 4 months ago

What do you think Aditya? Can the Universe be a holographic projection? And if it is then all of us are mere illusions created by the brain. And if we are illusions then our brains too are imaginary, we are relatively Imaginary. For me other people are illusions and i am illusion for others

- 7 years, 4 months ago

they think this because of the famous double slit experiment i think. Electrons and other subatomic particles seem to behave differently when unobserved i.e from particle like behaviour to wave like behaviour. Imagine when you look in front of you you see solid matter but as soon as you turn around the object dissipates into wave form.

- 7 years, 4 months ago

Yes Sakura you are right upto some extent.

- 7 years, 4 months ago

Sudarshan i am not saying that Universe is a holographic projection i just only told that some scientists think.And as per my views i upto some level i agree them because of my personal practical experiences and my personal thinking also.

- 7 years, 4 months ago

Well,there is one explanation.We know that dark energy is the repulsive force in this universe which is why the universe is expanding as the galaxies are moving farther apart.But this is the only thing we know about dark energy.Let me come to the reason.We all are more familiar with gravitation.We know that gravitation is the weakest fundamental force of nature.But we also know that gravitation between two or more bodies depends upon the mass of the bodies involved and the distance between them.And not only that,this gravittational force is an attractive force,unlike the dark energy,which is a repulsive force.So,whether an area of the universe is expanding or contracting depends upon whether gravitation will win or dark energy will win.So,when we compare the solar system,though the mass is very small when compared to the whole universe,the distance is very small and so the gravitation wins over the dark energy.This is also the case while comparing a galaxy and its inner components.But when we take the case of two or more galaxies,they are very far apart i.e,. the distance is very large.So,the dark energy wins over the gravitation,and so they move away from each other.I hope this may have answered your question.

- 7 years, 4 months ago

Yes Akarsh you are saying right.

- 7 years, 4 months ago

The Universe is expanding. ... The only thing expanding is space itself. ... away from each other as there's more bread expanding between them. ... In this case, the pull of gravity between the Milky Way and ... And if it's true, then the space between stars, planets and even atoms will expand in the far future.

- 7 years, 4 months ago

All right, I searched around about Boltzmann Brains and came across this article:http://www.preposterousuniverse.com/blog/2013/08/22/the-higgs-boson-vs-boltzmann-brains/ which states that "Ah, but things are a bit more complicated than that. We now know that the universe is not only expanding, but also accelerating. The simplest explanation for that — not the only one, of course — is that empty space is suffused with a fixed amount of vacuum energy, a.k.a. the cosmological constant. Vacuum energy doesn’t dilute away as the universe expands; there’s nothing in principle from stopping it from lasting forever. So even if the universe is finite in age now, there’s nothing to stop it from lasting indefinitely into the future." So in essence its Einstein's cosmological constant at play, permeating the universe. The effect is not 'diluted' when space time is stretched, so it continues to expand, and it accelerates. Like a wave that builds up and does not lessen over time/distance.

- 7 years, 4 months ago

dark matter

- 7 years, 4 months ago

No Kauts Dark matter is the force that keeps the universe binded but it also fails in against of dark energy

- 7 years, 4 months ago

gravity is repulsive at large distance and attractive in short distances

- 7 years, 4 months ago

from my point of view, the whole universe rotates in a way all together in one direction-from large scale pont of view- and as it rotates it experince a force that cause every part of it to move toward the outward. And the sun it self is not fixed in a fixed position, however , relative to us the sun is fixed and we are moving. What I am saying is that everything is moving and on the large scale point of view the universe planets and whatever matter is there tends to be moving around, but maybe from a small scale point of view we tend to neglect that bcz its small.

thats what i think, I hhaven't really searched for anything yet.

- 7 years, 4 months ago