Post on 11-Jan-2016
description
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)
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