Universe smaller than a Proton.

[Himanshu]

How do scientists assert the daring conclusion, that is overwhelmingly impossible even to speculate about, that the entire universe that contains billions of galaxies with billions of stars in each one of them along with dark matter and dark energy that make up 95% of our universe was once in diststant past so small that is occupied a region of space that is smaller than the volume of a proton?


Now I agree that the size of the universe must have come from the calculation from the physical theories. But is there an intuitive understanding for this?

 

[DaveC426913]

Hard as it is to conceive of the universe starting that way, the physical observations point very strongly in that direction.

All that matter and energy in the universe is flying apart at just the right speed, and coooling at just the right rate. If we switch the cosmic movie projector in reverse, and apply what we DO know about matter and energy, we see everything winding back to such a point.

The preponderance of evidence from different sources (such as Olber's Paradox, to name just one) leaves little room for other ideas about the origin of our universe.

You (i.e. you) could happily throw away all theories ever put forth and start from scratch, with only the observations. You would very likely come to the same conclusion about the origin of the universe.

It less about theories than it is about one virtually inescapable conclusion, the theories are more about refinement of the basic idea.

 

[rbj]

Hard as it is to conceive of the universe starting that way, the physical observations point very strongly in that direction.

All that matter and energy in the universe is flying apart at just the right speed, and coooling at just the right rate. If we switch the cosmic movie projector in reverse, and apply what we DO know about matter and energy, we see everything winding back to such a point.

but from what we are observing of the universe and its expansion, and considering the scale of these observations (zillions of lightyears), couldn't it be just as plausible that all of the universe "exploded" (i know that's the wrong word for this in the Big Bang) from a space as large as a solar system, or a star, or a planet, or a basketball? how do we infer that it goes back to the Planck scale (which is a fukuva lot smaller than a proton)?

 

[DaveC426913]

couldn't it be just as plausible that all of the universe "exploded" (i know that's the wrong word for this in the Big Bang) from a space as large as a solar system, or a star, or a planet, or a basketball?

But then we would have to explain why it's that size, why it expanded from that point, and what it was like before that point.

Especially that last one. How did it get to be the size of a planet or a basketball and expanding rapidly? The most logical answer as to how it got to this size is that it was even smaller and expanded to that size. There's no reason in physics that prevents it from benig smaller than a basketball.