720 reputation
616
bio website
location United Kingdom
age
visits member for 3 years, 2 months
seen 10 hours ago

Manual worker. Live in England. Formal scientific education "peaked" many years ago when I failed maths and chemistry A-level and scraped a bare pass in physics. Still enjoy pottering about in maths and physics.


Apr
24
asked Why do objects follow geodesics in spacetime?
Apr
8
accepted Hubble time, the age of the Universe and expansion rate
Apr
8
comment Hubble time, the age of the Universe and expansion rate
thanks very much.
Apr
7
asked Hubble time, the age of the Universe and expansion rate
Apr
7
comment How does the critical density decide the fate of the Universe?
of course! Funny how some things are obvious when they're pointed out. I was getting the strangest results trying to feed my equation into the WolframAlpha differential equation calculator. Thanks.
Apr
7
accepted How does the critical density decide the fate of the Universe?
Apr
6
comment How does the critical density decide the fate of the Universe?
thanks. Ryden uses parametric equations to get her big bang to big crunch graph of a matter only universe on p87. Just out of interest, do you know why I can't set the first term on the rhs of the Friedmann equation to a constant $C$, $k=+1$, $c=1$ to obtain $$\left[\frac{1}{R}\frac{dR}{dt}\right]^{2}=C-\frac{1}{R^{2}}$$ $$\frac{dR}{dt}=\left(R^{2}-1\right)^{1/2}$$ but when I try to solve this, I don't get the nice Big Bang to Big Crunch graph that Ryden does?
Apr
5
asked How does the critical density decide the fate of the Universe?
Apr
3
comment Very basic question about empty universe
Thanks for that. I looked at the paper - superluminal galaxies are not travelling faster than light in our or any other observer's inertial frame so don't contradict STR. That is amazing.
Apr
3
accepted Very basic question about empty universe
Apr
2
revised Very basic question about empty universe
added 92 characters in body
Apr
1
asked Very basic question about empty universe
Apr
1
comment Why doesn't dark energy vary with time?
thanks for that
Mar
31
accepted Why doesn't dark energy vary with time?
Mar
30
asked Why doesn't dark energy vary with time?
Mar
9
comment Finding the correct units for the energy-momentum tensor?
Thanks, though I find it hard to visualise something (the four components of four-momentum) flowing across a surface of unit time per unit time?
Mar
9
accepted Finding the correct units for the energy-momentum tensor?
Mar
9
comment Finding the correct units for the energy-momentum tensor?
Thanks. Just to be clear. Are you saying the units of area are $m^{2}$ even when the surface is time, ie for all four $T^{\mu0}$ components? Than makes sense if time is measured in $ct\equiv m$ units. If that's right, then that's clearer.
Mar
9
asked Finding the correct units for the energy-momentum tensor?
Mar
6
comment Still trying to understand gravitational potential and Poisson's equation?
self taught I'm afraid, and I know it shows. Last time I studied physics and maths was many years ago at school when I took my A levels (here in the UK). I've been trying to teach myself GTR for the last year or so, starting with the free online Leonard Susskind lectures. It's hard but fun.