#### Izack

##### Non sequitur
Oh wow, completely missed the word 'orbit' in that question. Oops. :embarrassed:

#### jroly

##### Donator
Donator
I have a question, will the universe continue to expand indefinitely or will it eventually collapse.

I thought about it and even though it would take an extremely long time, all the matter in the universe should eventually collapse in on itself into a blackhole.

My thinking is assuming a universe with only the earth contained within it, if you shot an object from the surface of the Earth at the speed of light, the inverse square law of gravity will continue to exert a pulling force on the object for infinity until eventually it will be pulled back towards the Earth.

Only way this would not happen is if there is a minimum amount of a gravity amount possible in the Universe and after a certain distance the gravity effect is completely 0.

Why are physicists unable confirm either continual expansion or eventual collapse? What are the reasons behind it, is dark matter the cause of the confusion?

#### RisingFury

##### OBSP developer
My thinking is assuming a universe with only the earth contained within it, if you shot an object from the surface of the Earth at the speed of light, the inverse square law of gravity will continue to exert a pulling force on the object for infinity until eventually it will be pulled back towards the Earth.

Your thinking is wrong. If you shoot an object away from Earth at more than its escape velocity of about 11.2 km/s, it will not come back down. The speed is so great that even over the course of infinite distance, the object wouldn't be pulled back.

The same goes for the universe. There is a speed at which it would escape its own collapse. But more than that, while gravity is pulling the universe together, in on itself, we've actually measured that it's expanding and accelerating its expansion. We don't know what's causing it.

It's like throwing an apple up, but not only does the apple not return, it accelerates upwards...

#### Proximus

##### New member
Does the same force that is expanding the universe actually affect atomic particles as well?
For example is the distance between protons and electrons getting bigger but we only notice the effect over the massive cumulituve distances between galaxies?

Or can gravity overcome the expansion force where two masses are closer together?

What could possibly be the reason behind such an expansion?

#### Urwumpe

##### Not funny anymore
Donator
Does the same force that is expanding the universe actually affect atomic particles as well?

No. The expansion is at such scales much weaker than the forces that keep particles together.

The expansion is just 74 km every second for every Megaparsec of distance. Which is an unbelieveable large distance. Pluto is about one light day away from Earth. One Megaparsec is 3260000 light years, or about 1,000,000,000 light-days, very coarsely calculated.

This means the distance Earth to Pluto would just grow by about 74 micrometers every second. It would take a few thousand years for this distance growing by a single meter.

#### RGClark

##### Mathematician
No. The expansion is at such scales much weaker than the forces that keep particles together.
The expansion is just 74 km every second for every Megaparsec of distance. Which is an unbelieveable large distance. Pluto is about one light day away from Earth. One Megaparsec is 3260000 light years, or about 1,000,000,000 light-days, very coarsely calculated.
This means the distance Earth to Pluto would just grow by about 74 micrometers every second. It would take a few thousand years for this distance growing by a single meter.

I didn't check your conversion for a Pluto distance, but there are 60*60*24*365 = 31,536,000 seconds in a year. Then taking the 74 microns per second number at face value, this would be 2,300 meters, 2.3 km, in a single year.

Bob Clark

#### Urwumpe

##### Not funny anymore
Donator
I didn't check your conversion for a Pluto distance, but there are 60*60*24*365 = 31,536,000 seconds in a year. Then taking the 74 microns per second number at face value, this would be 2,300 meters, 2.3 km, in a single year.

Bob Clark

74E-6 meters * 31E6 ... 74 * 31 meters per year. Yep, had somehow estimated with 74E-12 meters instead. Not sure why.

#### Quick_Nick

##### Passed the Turing Test
Donator
I didn't check your conversion for a Pluto distance, but there are 60*60*24*365 = 31,536,000 seconds in a year. Then taking the 74 microns per second number at face value, this would be 2,300 meters, 2.3 km, in a single year.

Bob Clark

That's right. Pluto isn't a light-day away though.
(0.2-0.3 light-days, not too far off)

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#### Urwumpe

##### Not funny anymore
Donator
That's right. Pluto isn't a light-day away though.

Its about the same magnitude, I used Pluto as reference there because most people with some spaceflight interest can still put this distance into relation to others. Pluto is 50 AU away at apohelion, which is only about 7 light-hours or 1/3rd light day away from the sun. But I had no interest in using exact math for showing how slowly something grows - the magnitudes of the numbers had been more interesting there.

The growth rate of one light-day space is still about correct.

#### Buck Rogers

ITN

After throwing one maritime cliche analogy about spaceflight after the other out the window over the years a while ago I read about the [ame="http://en.wikipedia.org/wiki/Interplanetary_Transport_Network"]ITN[/ame]. Finally one I can keep! (Not that I'm too bothered that space is not like sea travel, that would be kinda boring, and find it more interesting that there are new and different challenges ahead.) One floats around the solar system like on ocean currents! But very, very slowly!
My question is, could one ,using solar energy; as in no additional fuel, sail down these corridors fast enough to make it viable atleast for logistics (unmanned transport of bases etc..). Can one replicate this in Orbiter? Is the physics code detailed enough!

#### Proximus

##### New member
So essentially many many many years in the future, it will not be possible to see other galaxlies or even stars as they have all sped past the speed of light away from us?

A bleak prospect indeed

:ninja:

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#### MattBaker

##### New member
Well, a hundred trillion years from now all stars have exhausted their fuel and no new stars are formed so everything left are stellar remnants like white dwarfs, neutron stars, black holes, brown dwarfs.

I hope we have good batteries for desk lamps until then...

#### Quick_Nick

##### Passed the Turing Test
Donator
So essentially many many many years in the future, it will not be possible to see other galaxlies or even stars as they have all sped past the speed of light away from us?

A bleak prospect indeed

It will one day be completely impossible for the inhabitants of any galaxy to know that any other galaxies exist or ever existed. Since that will be in 150 billion years, I wouldn't be surprised if something more interesting happens well before then to the entire galaxy or universe.

#### Donamy

Donator
Beta Tester
I would rather not think about it, ...it makes my head hurt.

#### Kyle

##### Armchair Astronaut
Could someone explain why inflation works as a solution to the "Horizon Problem?"

#### Quick_Nick

##### Passed the Turing Test
Donator
Could someone explain why inflation works as a solution to the "Horizon Problem?"

"Prior to the inflation the entire universe was small and causally connected; it was during this period that the physical properties evened out. Inflation then expanded the universe rapidly, "locking in" the uniformity at large distances."
-Prof. Wikipedia

#### Proximus

##### New member
Could the aether be the cause of the expansion of the universe?
If in fact the Aether exists at all, it could be slowly expanding away from us delaying the light that we see from other galaxies and stars giving us the illusion of an ever expanding universe?

:hmm:

#### Urwumpe

##### Not funny anymore
Donator
Could the aether be the cause of the expansion of the universe?
If in fact the Aether exists at all, it could be slowly expanding away from us delaying the light that we see from other galaxies and stars giving us the illusion of an ever expanding universe?

:hmm:

No, for two very simple reasons:

1. What is Aether and which measurable properties does it have? The old concept of Aether was defined and was found to be wrong. So you need a new definition.

2. The cause of the expansion is actually known. The phenomena is called vacuum decay and is a bit longer to explain for me, since I am no theoretical physicist and would need to spend a weekend over papers and books before finding some simpler words for it.

In very likely wrong words: You can imagine the universe as an inside out black hole (which has some interesting implications on how a black hole could look from the inside, should you ever reach it), and this inverted black hole grows by its space reaching lower energy levels (opposite to a black hole from the outside, which grows by increasing energy levels).

Now can please a theoretical physicist stop me before I say something really wrong? Thank you.

#### Donamy

Donator
Beta Tester
For one thing, ... there aren't any.

#### Zatnikitelman

Does that cost account for inflation?

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