Skip to main content


Showing posts from March, 2018

Is Quantum Cosmology Serious Physics?

Alexander Vilenkin, one of the major contributors to the field.

Quantum cosmology is in an incredibly infantile and esoteric branch of theoretical physics; with a whole series of practical and conceptual problems. Chris Isham says because of these, no one is really sure whether quantum cosmology is even a valid branch of science, or whether the whole project is just misconceived.

The unique claim of quantum cosmology isn't just that quantum theory applies to things within the universe but to the universe itself, particularly when the universe was small enough at the Big Bang.

The first and third most popular posts I wrote on this blog were about the Hartle-Hawking and Vilenkin tunneling from "nothing" models, both of which are proposals for a quantum state of the universe (quantum cosmologies). Having read over these again and in light of Hawking's death I thought it might be useful to explain and contrast these in a single post, a bit more.

If I throw a sharpie acro…

Hey AronRa, Please Stop Talking Nonsense About Physics!

A couple of days ago I came across "Everything from Nothing" a video by AronRa, he's a science popularizer on the internet who decided to discuss the origin of the universe. It basically starts out in a tirade against religion and then essentially endorses a badly misrepresented version of Lawrence Krauss. A lot of the claims being made in the video are outright false but when they're sold to us by someone who may have a working knowledge of biology, or who drops technical language (a lot of which he himself doesn't understand), it makes it tempting for some people to be misled by what he's saying.

I'm going to go through a few of the claims he's made ignoring all of the over the top, anti-religious crap.

It's difficult to know if some of these are slip-ups or genuine ignorance. They're not the kind of mistake anyone with an actual understanding of the topic would make. He equivocates for example between "expansion" and "inflat…

Chaotic Inflation Violates Penrose-Hawking Singularity Theorems

Claim: ChaoticInflation can avoid the implications of the Penrose-Hawking singularity theorem 

In the last post I talked about the Penrose-Hawking singularity theorem, and the conditions under which the theorem can be violated. One of the assumptions that Penrose and Hawking made was the strong energy condition, which states

This is true when the cosmological constant of the universe is zero, this has been a major weakness in their argument because neither the theorem nor the Friedman equation can tell us what the material or field content of the early universe was which has a direct affect on the pressure, mass density and expansion rate of the early universe. Under the second Friedman equation

If the pressure of matter p is positive then the rate of expansion is either decreasing or the rate of contraction is increasing but if the pressure is negative and the early universe is pervade by tension (a negative gravitational force) rather than normal pressure, then the expansion rate w…

Penrose-Hawking Singularity Theorems

Between 1965 and 1970, Penrose and Hawking proved a set of three singularity theorems together, all of which make varied assumptions and have different starting points. ** when a cosmologist refers to these as the "Penrose-Hawking Singularity theorem" they're mostly interested in the last theorem, which was proven in 1969 in a famous paper titled "The singularities of gravitational collapse and cosmology". This is the most powerful formulation of the Penrose-Hawking theorem, and it's what I'll talk about here.

Defining what a singularity is can be difficult. In some places you may find that a singularity appears in a model when the null or timelike geodesics cannot be extended indefinitely; that is to say, if the paths of photons or particles with mass terminate at some finite time into the past. Penrose-Hawking try to show that this condition obtain in our universe given some general features of spacetime.

Physically, interpreting that would mean that…