Friday, December 8, 2017

Four distinct responses to the cosmological constant problem

One of the biggest problems in theoretical physics is to explain why the cosmological constant has the value that it does.
There are two aspects to the problem.
The first problem is that the value is so small, 120 orders of magnitude smaller than what one estimates based on the quantum vacuum energy!
The second problem is that the value seems to be finely tuned (to 120 significant figures!) to the value of the mass energy.

The problems and proposed (unsuccessful) solutions are nicely reviewed in an article written in 2000 by Steven Weinberg.

There seem to be four distinct responses to this problem.

1. Traditional scientific optimism.
A yet to be discovered theory will explain all this.

2. Fatalism. 
That is just the way things are. We will never understand it.

3. Teleology and Design.
God made it this way.

4. The Multiverse.
This finely tuned value is just an accident. Our universe is one of zillions possible. Each has different fundamental constants.

It is is amazing how radical 2, 3, and 4, are.

I have benefited from some helpful discussions about this with Robert Mann. There is a Youtube video where we discuss the multiverse. Some people love the video. Others think it incredibly boring. I think we are both too soft on the multiverse.


  1. Maybe see this first

    1. Thanks for the helpful link.
      I think Bee is being a bit pedantic. The point remains that the cosmological constant is "fine tuned" and is many orders of magnitude smaller than a rough estimate from quantum field theory. Sure, we can debate whether it is a "prediction" and exactly what "theory" is predicted it. But the problem remains that there is a big problem!
      The fact that we don't even have an uncontentious way to "predict" the value from a well-defined theory actually further highlights how big the problem is.

  2. I've never been comfortable with the "fine-tuning" phrasing. Someone once compared the situation to having a pool the size of 13 Olympic-size swimming pools holding a single molecule of water, and then claiming that that pool was fine-tuned for holding water!