Re: Evidence for "Big Bang Theory"

Arun Gupta (
Tue, 18 Apr 1995 21:39:50 GMT

In article <>,
Robert Roosen <> wrote:
>I believe you've got the cart before the horse on this point. As
>Burbidge pointed out, "If you interpret the redshifts as expansion
>redshifts (subtly different from "explosion" redshifts), then the
>universe is expanding."
> Definitely, interpreting the redshifts in this way led to various
>expanding universe theories, including the big bang.
> This is an example of people discovering their assumptions and
>treating them as something new.

No, I don't think so. There is a whole series of methods of distance -
determination in astronomy. Each step on the distance scale depends on
the previous ones, and of course, errors and uncertainties accumulate,
but the measurements are pretty good, all considered.

When you observe a celestial object, what you can measure is its observed
brightness in various parts of the electromagnetic spectrum, and its
position in the sky.

The position in the sky is relevant for distance measurements only for
nearby stars, where parallax measurements can give a direct measure of

The other thing you can try to do then is to find something of which you
know the actual brightness, and measuring the observed brightness, you
can figure out how far away it is. (Of course, you have to worry about
things like absorption of light by interstellar dust; but such things
modify the spectrum of light from the object in definite ways that you
can try to work out).

There are a whole bunch of standard candles. Cepheid variables are stars
whose brightness (luminosity) changes periodically, and the time period is
related to their absolute luminosity (or actual brightness). By observing
an entire set of them at roughly the same distance (say in the Magellenic
clouds) you can calibrate the brightness-period relationship and then
measure distances in terms of the here-to-Magellenic-cloud-distance whenever
you can spot one of these stars (i.e, measure the apparent brightness, the
time period, you know then what the actual brightness is, and how much
it has attenuated in its travel from there to here, and so the distance.)

The Hubble telescope makes it possible for the first time, to resolve stars
in the Virgo cluster of galaxies (or so I understand) and so allows a pretty
good distance determination to that cluster.

Galaxies are visible for much greater distances than stars, and so if you
can identify a representative galaxy, (as you can in the Virgo cluster),
you can then measure distances in terms of distances to the Virgo cluster.
Before Hubble, the distance to the Virgo cluster wasn't very certain. That's
where most of the uncertainty in the distance to very distant galaxies
came from. But distances to galaxies in units of distance to the Virgo
cluster are good and have been good for a long time.

It is in the units of the last mentioned scale, to the Virgo cluster, that
astronomers measure both the quite-uniform expansion of the universe (i.e,
the distance-redshift relation), and the deviations from it (the Great
Attractor, etc.)

-arun gupta