This week's CQW is a guest post by Phillip Helbig:

Read the question then, without calculating or estimating anything, first make a quick guess as to the result. Then work out the result (an order of magnitude or two is close enough).

Neglect gravity and other types of interactions and imagine the entire observable universe being compressed into a ball (rather like a movie of the expanding universe played in reverse). There are about a hundred billion stars per galaxy on average, and at least a hundred billion galaxies in the observable universe. Or think of the Hubble Deep Field, which has the angular size of a small rice grain held at arm's length, full of galaxies - and this was intentionally chosen because it was apparently empty! The sky is about 25 million times larger.

What is the size of the ball when the density is equal to the density of nuclear matter?

## Wednesday, November 25, 2015

## Wednesday, November 18, 2015

### let it rip!

what property of dark energy would lead to an infinite scale factor in a finite time in the future? would a cosmological constant be able to do this?

## Wednesday, November 11, 2015

### no big bang

can you construct a cosmological model with an asymptotically constant scale factor in the past, such that the age of the universe would be infinite and no big bang would have occurred? what would be a stable construction?

## Wednesday, November 4, 2015

### vacuum fluctuations

what's wrong with this argument? a moving observer would see quantum fluctuations in e.g. the electromagnetic field blueshifted from the forward and redshifted from the backward direction and would be able to determine the velocity by the amplitude of the motion dipole.

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