L2 - the Big Bang

What is Obler’s Paradox?

• There was no explanation for why it is dark at night

• The black background between stars suggested either the universe is finite or that the light emitted from stars is intercepted by dark matter

Who came up with the ‘cosmic egg’ theory? When?

• Georges Lemaître

• 1920s

What was the ‘cosmic egg’ theory?

• Lemaître envisioned that there was a concentrated entity

• It exploded and thew matter away from each other and into an expanding universe

• The force of the explosion prevents the gravitational pull from drawing the matter towards the centre of the universe

What did Hubble observe?

• Hubble reported a shift towards the red spectra of light reaching us from distant galaxies

  • Meant the galaxies were speeding away from ours at incredible speeds

  • This observation provided evidence for the expanding universe theory.

What do the missing wavelengths mean in this image?

The missing wavelengths indicate that certain elements are being absorbed by the gas in the atmosphere of the star or galaxy, which can reveal information about its composition and movement.

Explain ‘red shift’

• The further away things are, the closer to red they become

• The light is being stretched out because the light source is moving away from us - resulting in stars and galaxies appearing red.

Explain the Doppler Effect in relation to a police car

• As the police car moves towards you, the sound waves compress and the frequency increases, raising the pitch.

• As the police care moves away, that stretches the sound waves, increasing wavelengths and lowering pitch

Who discovered Cepheid stars?

Henrietta Levitt

What are Cepheid stars?

• Henrietta Levitt noticed stars vary in brightness

• Different stars ‘pulse’ at different rates but the rate of pulsation correlated with how bright the star was.

• Relationships between the luminosity of stars and the rate it pulses are a universal, fundamental property of stars

How can Cepheid stars be used to calculate the distance of stars?

• From the blinking rate, the luminosity of a star can be estimated

• By comparing the luminosity with the intensity of light seen from earth, the distance of a star could be determined

How can Cepheid stars be used to calculate the distance of distant galaxies?

• For most distant galaxies, the galaxy appears only slightly larger than a nearby star, so no pulsing stars can be identified.

• Most galaxies are found in clusters

• Galaxies in very distant clusters have a similar spectrum of sizes and brightness as the ones in closer clusters

• Therefore, can use the same method to measure the distance of distant galaxies.

What does this graph tell us about the date of the Big Bang?

• The gradient tells us how long things have been moving apart for.

• Straight line provides solid evidence of Big Bang event, in which everything was created and started from the same place, and has been moving outwards at different velocities.

What is blackbody radiation?

• All objects above absolute zero emit radiation that is diagnostic of their temperature

• This radiation can be used to estimate the temperature of distant objects.

• The wavelength of emitted radiation decreases as the temperature increases.

How does temperature influence blackbody radiation?

• At very low temperatures, the radiation is not visible

• But, as temperature gets above a few hundred degrees C, the wavelengths start to enter the visible range, and the object glows a dull red.

Describe how blackbody radiation works in hot temperatures

• If an object is heated up, it will absorb some of that energy and reradiate that energy in the visible spectrum of red light (i.e. ‘red hot’)

• As that object gets hotter, there will be more of the visible spectrum of light emitted, so that the object will appear to be flowing white (i.e., ‘white hot’)

How was blackbody radiation discovered?

• An experiment on radio waves detected a signal which corresponds to a blackbody radiation/temperature of 2.73 kelvin.

• Initially thought it was noise.

• Later realised the detected radiation was a background signal/echo from the Big Bang

How does the present of Hydrogen and Helium provide further support for the Big Bang?

• The universe matter immediately after the Big Bang consisted almost entirely of only 2 elements: H and He

• The calculated 10:1 distribution is consistent with theoretical calculations of the Big Bang event and creations of matter

Why was dark energy hypothesised?

• 1990s Hubble telescope launched

• Scientists found the universe is still expanding and that expansion is speeding up

  • The contracting universe theory emerged following this.

• Things are moving apart, and the rate at which they do is accelerating: no one really knows why.

• Dark energy was hypothesised following this

What is dark energy (theoretically)?

• There has to be some sort of energy source that is strong enough to overcome the forces of gravitational attraction, which should be slowing down the expansion of the universe

• Dark energy, in theory, explains why the universe is still expanding at an increasing rate.