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New stars keep forming in the universe thanks to all the nebulae. Now, we need Hydrogen to form stars and there would a time when all the hydrogen will get exhausted, and no more star formation will take place, theoretically.

Will there practically be such a point of time? I guess there is no place where hydrogen is replenished after star formation. Or is there a feedback cycle?

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There is no feedback cycle, there will be a point where no stars form anymore, entropy dictates it. –  Eduardo Serra Jan 13 at 16:38
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@EduardoSerra To be more precise, there is a feedback cycle, but it is a negative feedback rather than a positive. Entropy is the law of decreasing returns essentially. –  called2voyage Jan 13 at 16:53
    
Who deleted my comment? There is a positive feedback cycle, which is very weak though. Protons and antiprotons can be produced in black hole evaporation as a side product. –  Alexey Bobrick Jan 14 at 13:11
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up vote 8 down vote accepted

Cosmic GDP has already crashed, as Peak Star was ~11 billion years ago. Cosmic GDP

According to Sobral et al's prediction, the future star production by mass will give only 5% of the stars in the universe today, "even if we wait forever." More theoretical predictions, such as this one, suggest that nebulae will run out of hydrogen on the order of $10^{13}$ years, while star formation will occasionally happen due to collision of brown dwarfs until somewhere on the order of $10^{14}$ years.

Of course, hydrogen itself may have a finite lifetime. The half-life of a proton is experimentally known to be longer than $10^{34}$ years, but it may still be quite finite.

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I thought I remembered reading something about this, but I couldn't remember where so I left it to someone else to answer. +1 –  called2voyage Jan 13 at 20:30
    
Very well explained. Exactly the answer I was looking for. Thank you! –  Ranveer Jan 13 at 22:02
    
@called2voyage Brian Cox did a nice show on this in his "Wonders of the Universe" series, but if I recall, he got a bit poetic in specifying the time frames involved. Basically a really really really long time. –  Robert Cartaino Jan 13 at 23:39
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Could it be that at after a certain time, the "dark matter" observed, will become hydrogen? Perhaps after such a long time, that we could never observe it close enough? –  frodeborli Jan 14 at 11:13
    
@frodeborli: If it were the case, that would shift the "no more new stars" date upwards. But there's no reason to believe it. –  Stan Liou Jan 14 at 11:18
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Given what we know of entropy and of the expansion of the Universe, the answer to your question is of course "Yes".

It will take a really really long time, however.

Edited to add:

"...a really really long time..." is an intentionally vague description. Whether it is 1010 or 101010 Hubble times is open to question.

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Could you be a little bit more precise and indicate, what do you really mean by really really long? Is it several Hubble times, or $10^{10}$ Hubble times, or $10^{10^{10}}$? –  Alexey Bobrick Jan 13 at 18:04
    
How precise do you need it, @AlexeyBobrick? –  Cyberherbalist Jan 13 at 21:54
    
As precise as possible :) –  Alexey Bobrick Jan 13 at 22:31
    
OK, @AlexeyBobrick, how about 983.98332 x 10^10 Hubble times? –  Cyberherbalist Jan 13 at 22:44
    
Nice, but not very convincing, sorry. –  Alexey Bobrick Jan 13 at 22:45
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