http://apod.nasa.gov/apod/image/1103/ngc5584_hstr.jpg

Dark Energy & Quasars

StarterWhat assumption did we make when calculating the age of the Universe?

Expansion of universe is constant

21/04/2023

Dark Energy & Quasars

• By the end of the lesson you should be able to:

• Understand the controversy about the accelerating universe and dark energy

• Understand properties of quasars

Accelerating universe• Expansion of the universe is not constant• Initially thought to be slowing down, due to

gravity. If so:– More distant objects would be seen to be

receding faster– Object would appear brighter than predicted

• Recent observations of Type I supernovae have shown they are dimmer than expected– Rate of expansion is accelerating

Dark Energy

• Cause of acceleration is unknown– Not matter or radiation

• One possibility is dark energy• Exerts an overall repulsive effect throughout

the Universe causing empty space to expand.– Opposes the force of gravity

• Universe composed of 73% dark energy and 27% matter (mostly dark matter)

Quasars

What are Quasars?

• Quasi-stellar objects• 1960s found at radio frequencies but now

found to emit all EM wavelengths• Extremely large red shifts of spectral lines– Recession velocities in excess of 4×107ms-1

• Some of the most distant objects in known universe

• Optically very faint from Earth• Due to inverse square law, amongst the

brightest objects in the Universe.• Can outshine rest of galaxy• Quasar 3C273 has a luminosity of 1040W– 20 trillion suns

• But smaller than a galaxy

Unusual properties of quasars

• Irregular variation in brightness from days to weeks to months

• Long jets of radiation– Due to electron, travelling close to the speed of

light, spiralling in the local magnetic field– Emitting bright beams radiation at all wavelengths

What are quasars?

• No one’s quite sure• Intensely bright

cores of active galaxies

• Supermassive black hole at the centre

• Gobble up stars at a rate of 10 solar masses a year

Question

• Quasar 3C 273 has a red shift, z = 0.16. • Using Hubble’s constant H0 = 73kms-1Mpc-1.• Determine how far away this quasar is from

the Earth in Mpc and ly

Answer

• v=cz = 3×105(kms-1)×0.16 = 4.8×104kms-1

• d=v/H0 = 1.9×107/73 =658Mpc• 658Mpc × 3.26 = 2.15 billion lightyears

• CFHQS J2329-0301: red shift, z=6.43

Question

• What is the recession speed of quasar CFHQS J2329-0301? How far away is it in ly?

• You can to use the relativistic equation for red shift as the quasar is travelling very, very fast.

𝑧=√ 𝑐+𝑣𝑐−𝑣

−1

• Hint: put c in kms-1 • v = 2.89×105kms-1

• d = 12.9 billion ly

• Galaxy UDFj-39546284: z = 10.3 • 13.2 billion light years