our dynamic universe

It’s complicated and cosmologists are not certain.  One of the issues is only being able to see about 4% of the mass in the universe – the stars, planets, gas and dust.  About 25% of the mass of the universe is Dark Matter.  It’s “dark” because it doesn’t emit light that enables us to see it.  Vera Rubin and Fritz Zwicky were the two astronomers who produced observations that led to the dark matter theory.

Vera Rubin measured star velocities in the Andromeda galaxy and plotted these against the star’s distance from the centre of the galaxy.  Knowledge of rotational speeds within our Solar System would predict a graph similar to curve A.  What she obtained was a relatively flat graph (B).

image from Quantum Diaries

The rotational speed of the stars in curve B are far too fast for the Andromeda galaxy to stay together.  The only explanation for the galaxy staying together was the presence of an awful lot of additional mass that couldn’t be detected.  This new mass was named dark matter.

Rubin talks about her discovery in this video.

Vera Ruben dark matter dark energy from mr mackenzie on Vimeo.

Zwicky had been looking at clusters of galaxies, rather than individual stars within galaxies.  He found something similar; the galaxies were swirling round at too great a speed and should fly apart. 

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You might remember that we looked at some paradoxes when we studied special relativity earlier this term.  Here is another situation where a paradox can arise.  The German astronomer Heinrich Olbers (1758–1840) asked why the night sky was dark.  At the time, astronomers believed that the Universe was both infinite and steady state (unchanging), so it seemed like a good question.

• Wouldn’t there be a star in any direction you chose to look?
• Shouldn’t the light from that star prevent the night sky from looking dark?

Well, the problem is that the Universe is not infinite because it is still expanding.  The Universe also isn’t steady state because it is… expanding.  It turns out that a question posed by a follower of the infinite, steady state model of the Universe is actually a decent piece of evidence in support of the Big Bang model of the Universe.

Watch these two videos and see how they chip away at the paradox and show how the answers to the question turn out to support the expanding Universe model.

As we worked through the diagram explaining the stages of the Big Bang model, we looked at a section of the diagram where the Universe was hot enough for nuclear fusion.  At this point, hydrogen nuclei were fusing together with other hydrogen nuclei to create helium nuclei.  As the Universe expanded, it cooled and further fusion was not possible.  As a result, we have a Universe with the same proportion of hydrogen to helium wherever we look: we find 75% hydrogen and 25 % helium.  This can only be the case if all of the helium was produced at the same place and the same time, i.e. in a very small, very hot Universe.

Hydrogen, helium and cosmic microwave background radiation from mr mackenzie on Vimeo.

introduction to the Big Bang from mr mackenzie on Vimeo.

Georges Lemaître’s theory of an expanding Universe, which has become known as the Big Bang, was supported by Hubble’s observations.  The expanding Universe idea was challenged by influential scientists who believed the Universe was both infinite (and therefore not expanding) and steady state (unchanging).  Supporters of the Big Bang idea needed to find other evidence that could confirm their model was correct.

The cosmic microwave background radiation (CMBR) is radiation left over from the big bang.  When the universe was very young, only 380,000 years old, just as space became transparent to light, electromagnetic energy would have propagated through space for the very first time.  At this stage in its development, the temperature of the Universe would have been about 3000K. Nowadays, the temperature of space has fallen to approximately 2.7 K (that’s 2.7 K above absolute zero!) and, using Wien’s Law, we can confirm that the peak wavelength of the electromagnetic radiation is so long that the background radiation lies in the microwave portion of the em spectrum.

The CMBR was first detected in 1964 by Richard Woodrow Wilson and Arno Allan Penzias, who worked at Bell Laboratories in the USA.

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In the 1920s, Edwin Hubble had access to the Hooker telescope on Mount Wilson, Los Angeles.  This was the largest telescope in the world at that time.  His first breakthrough was the discovery of a cepheid variable star in the Andromeda nebula.  This enabled him to calculate the distance to Andromeda and he quickly realised this was not a nebula but a galaxy outside the Milky Way.
This video follows his work.

Hubble – nebulae or galaxies? from mr mackenzie on Vimeo.

Hubble then turned his attention to other galaxies, looking for cepheid variable stars that would allow him to determine their distances from the Milky Way.  He used redshift to calculate their recession velocity and plotted a graph against their distance from us.

He found that the recession velocity (v) was directly proportional to distance (d).  We can express this relationship as

which is known as Hubble’s law, where is the Hubble constant.  Astronomers agree that the current value of the Hubble constant is

.

Since this is a SQA course, we need to convert the constant into SI units – giving

In this second video, Professor Jim Al-Khalili looks at Hubble’s work on measuring redshift for different galaxies and his discovery of an expanding universe.

Hubble’s discovery of the expanding universe from mr mackenzie on Vimeo.

Although he was American, Edwin Hubble transformed himself into a tea drinking, pipe smoking, tweed wearing Englishman during his time as a Rhodes Scholar at Oxford.  He probably wouldn’t approve of this last video.

It is said that when Hubble died, he left his collection of tweed jackets to Mr Jamieson-Caley.

Unfortunately, astronomers were not eligible for the Nobel Prize for Physics while Hubble was alive.  The rules have now been changed.