8.2 Red shift Flashcards
Galactic red shift
Usually, when an object emits waves, the wavefronts spread out symmetrically.
If the wave source moves, the waves can become squashed together or stretched out.
A moving object will cause the wavelength, λ, (and frequency) of the waves to change:
The wavelength of the waves in front of the source decreases (λ – Δλ) and the frequency increases.
The wavelength behind the source increases (λ + Δλ) and the frequency decreases.
This effect is known as the Doppler effect
The Doppler effect also affects light.
If an object moves towards an observer the wavelength of light decreases
This is known as blueshift as the light moves towards the blue end of the spectrum.
If an object moves away from an observer the wavelength of light increases
This is known as redshift as the light moves towards the red end of the spectrum.
Big bang theory
Around 14 billion years ago, the universe began from a very small region that was extremely hot and dense.
Then there were a series of explosions, which we call the Big Bang.
This caused the universe to expand, cooling as it does so, to form the universe we currently observe.
Each point expands away from the others.
This is seen from galaxies moving away from each other, and the further away they are the faster they move.
As a result of the initial explosions, the universe continues to expand.
Evidence for the Big Bang theory
By observing the light spectrums from supernovae in other galaxies there is evidence to suggest that distant galaxies are receding (moving further apart) ever faster.
These observations were first made in 1998.
The light spectrums show that light from distant galaxies is redshifted, which is evidence that the universe is expanding – the galaxies are moving away from us - and, indeed, each other.
This is what happens in an explosion.
Matter is first densely packed and as it explodes it, it moves out in all directions getting further and further from the source of the explosion.
Some matter will be lighter and travel at a greater speed, further from the source of the explosion.
Some matter will be heavier and travel at a slower speed, closer to the source of the explosion.
If you were to travel back in time and compare the separation distance of the galaxies they would become closer and closer together until the entire universe was a single point.
If the galaxies were originally all grouped together at a single point and were then exploded we would see a similar effect.
The galaxies that are moving fastest would move the furthest - the distance they move would be proportional to their speed.
The galaxies that are moving slowest would move the least.
Dark energy and Dark matter
Over the past two or three decades, astronomers have gathered plenty of evidence helping us to understand the universe much better.
However, there are still many observations that are still not understood, including:
Evidence from the rotation and motion of galaxies suggests that there is much more matter in the Universe than we are able to account for.
This unseen matter is given the name dark matter and its nature is not currently known.
Measurements of the expansion of the Universe using supernovae suggest that the Universe is starting to expand at a faster rate.
It has been suggested that this accelerated motion is caused by dark energy pushing everything in the Universe apart.
If gravity is present everywhere in the Universe, it might be expected that everything will one day become closer together, but that is the opposite of what we see.
This is another reason why dark energy is a possible solution which could be counteracting gravity and expanding the Universe instead.
Expanding universe
The diagram also shows that the light coming to us from distant galaxies is redshifted.
The lines on the spectrum are shifted towards the red end.
This indicates that the galaxies are moving away from us.
If the galaxies are moving away from us it means that the universe is expanding.
The observation of redshift from distant galaxies supports the Big Bang theory.
Another observation from looking at the light spectrums produced from distant galaxies is that the greater the distance to the galaxy, the greater the redshift.
This means that the further away a galaxy, the faster it is moving away from us.
