# The Naked Scientists Forum

### Author Topic: What was entropy at the big bang?  (Read 1734 times)

#### thedoc

• Sr. Member
• Posts: 511
• Thanked: 12 times
##### What was entropy at the big bang?
« on: 17/10/2014 09:30:01 »
Nikhil Teli asked the Naked Scientists:

According to second law Entropy of universe is continuously increasing.
So at the start of Big-Bang entropy should be zero.

Also temp. Of universe is continuously decreasing because it is expanding continuously.
So at start of Big-Bang temp. Should be maximum.

So finally, At start of big-bang
Temp should maximum
& entropy should zero.

But according to 3rd law
Entropy is zero @ absolute zero temp...
What is this matter?

What do you think?
« Last Edit: 17/10/2014 09:30:01 by _system »

#### JohnDuffield

• Sr. Member
• Posts: 488
• Thanked: 1 times
##### Re: What was entropy at the big bang?
« Reply #1 on: 17/10/2014 14:43:52 »
According to second law Entropy of universe is continuously increasing. So at the start of Big-Bang entropy should be zero.
That's what people tend to say, but strictly speaking it's only true for a "system" that stays the same size. Take a look at this Princeton article which is based on a Wikipedia article. It says entropy is a measure of the energy not available for useful work. In the simplest terms, entropy is "sameness". If the energy density of the universe is the same everywhere, there is no available energy, and you can't do any work. This is what people refer to when they talk about the heat death of the universe, where entropy is said to very high, whilst energy density is uniform and low. But imagine an early universe where energy density was very high, but uniform. If that universe stays the same size, there is no available energy, and you can't do any work. In this respect the entropy of the early universe was also high.

Also temp. Of universe is continuously decreasing because it is expanding continuously. So at start of Big-Bang temp. Should be maximum.
Again, that's what people say. But temperature is a measure of average kinetic energy or motion. And if nothing is moving, there is no temperature. The temperature is absolute zero.

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: What was entropy at the big bang?
« Reply #2 on: 17/10/2014 18:04:59 »
No temperature needs a interaction between what we define as matter (rest mass) and radiation. Take away any of those and temperature won't exist experimentally. And when it comes to the last stand, experiments are all.

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: What was entropy at the big bang?
« Reply #3 on: 17/10/2014 18:08:33 »
So, radiation acting on radiation will never be 'temperature' if you get my drift? But it is possible that a symmetry break can create rest mass, although I don't know how. Then again, maybe you need both 'created'.

#### The Naked Scientists Forum

##### Re: What was entropy at the big bang?
« Reply #3 on: 17/10/2014 18:08:33 »