Recent content by DAH

You're probably right and I would think all methods are challenging especially at very high redshift because the galaxies were much younger and smaller than typical nearby ones.

I once asked the same question, how do we know if the distant galaxies are accelerating now since the light we observe here and now is from billions of years ago? Then I found that cosmologists used methods such as measuring the apparent size of galaxies v redshift, otherwise known as the...

I was thinking an apple cut in half with the seeds in the middle. :smile:

I haven't done much astronomy myself but I'd say thats pretty impressive to capture an asteroid within a month of observing. :thumbup:

In a spatially flat universe the Hubble parameter is given by $$H^2=\frac{8\pi G(\rho_m+\rho_r)}{3}+\frac{\Lambda c^2}{3}$$ ##\rho_m## and ##\rho_r## decrease with time and ##\Lambda## stays constant, so the Hubble parameter decreases with time. In the remote future ##\rho_m## and ##\rho_r##...

I like to read threads like this one and they seem to get a lot of attention from the regulars here as well. I think the OP is getting confused with the difference between seeing events and observing events. Seeing an event is when the light from that event reaches your eye or telescope in this...

When observing galaxies there are three distances to consider, the proper distance at emission, the co-moving distance and the distance the light has traveled to reach us. The light travel distance is always somewhere between the emission distance and co-moving. All these can be calculated using...

When we observe the most distant galaxies, then we are seeing them at the time of emission when the radius of the observable universe was much smaller. A galaxy with a redshift of 20 will have a proper distance at emission of about 2 billion light year, however the co-moving distance (the...

I agree that she's wrong to say that the twin effect is solely due to acceleration, but I think what Sabine might be saying is that: Given two time-like separated events, where observers are present at both events, then the proper time measured along the world-line of a non-inertial observer is...

The way I see it in SR ##c=1## but it still has units ##\text{m/s}## so that ##ct## has units of meters, the same as the spatial coordinates. The relative velocities in the transformations will always be less than 1 or a fraction. For time dilation in one dimension, if two ticks of a watch are...

I think you're right, its the accuracy of the observations that are better for the precession of Mercury.

The book is Relativity, Gravitation and Cosmology. Author: Robert J.A. Lambourne Consultant author for the chapter is: Aiden Droogan You can find it here:https://is.muni.cz/el/ped/podzim2015/FY2BP_KOSM/Lambourne_Relativity__gravitation_and_cosmology.pdf Chapter 7 page 204

Its a level 3 text book as I study part time with the OU in the UK, but its only an introduction to relativity so not advanced GR. I just re read the section on precession and it also states that you can ignore the mass of the planet so there was no need for me to include ##M+m##. The...

In my book it gives ##6\pi GM## in the numerator but also states that ##M## is the total mass of the system. I suppose with the Sun totally dominating then just plugging in the Suns mass won't really alter the final calculation. Thanks.

Thanks for clarifying. I was surprised you didn't say radians for the OP since it would be easy enough to convert to seconds of arc per century as @vanhees71 shows. The OP probably already knew this anyway. :smile: