Cosmology

What is the Doppler effect?

The compression or spreading of waves that are emitted or reflected by a moving source.

What does the Doppler effect do to star line spectra?

Causes them to be shifted towards the blue end of the visible spectrum when they move towards the earth or towards the red end of the spectrum when they move away from the earth.

What is an eclipsing binary star system?

where two stars orbit around a common centre of mass with their orbital plane in the Earth's line of sight. when observed from earth the stars cross in front of each other as they orbit.

How can eclipsing binary star systems be identified?

from their characteristic light curves

What does a greater degree of red shift mean?

The faster the galaxy is moving away from earth. the furthest galaxies appear to be redshifted the most and are receding the fastest.

What is Hubble’s Law?

The recessional velocity of a galaxy proportional to its distance from earth. It can be expressed mathematically as: v=Hd where v= recessional velocity of an object, H= hubble constant, d= distance of object from earth. In terms of red shift this can also be written as z=Hd/c

What does Hubble’s law show?

The further away a galaxy is from the Earth, the greater the red shift, and the faster it is moving away. The closer a galaxy is to the Earth, the smaller the red shift, and the slower it is moving away.

what does the graph of Hubbles law look like?

what can Hubbles law be used for?

to find the time expansion began and hence the age of the universe, to do this we must assume all points in the universe were initially together, the recessional speed of a galaxy is and has always been constant.

what does Hubbles constant represent?

the rate at which the universe is expanding, if the universe has been expanding at a constant rate since the big bang then age of the universe = 1/H

Whate are the three key prices of evidence for the Big Bang theory?

Galactic red shift and Hubble’s law, Cosmic Microwave Background radiation and the relative abundance of hydrogen and helium.

How does galactic redshift evidence the Big Bang theory?

light spectra from distant galaxies shows redshift due to the expansion of the universe. observations also show the further away a galaxy is the greater the redshift and the faster its recession speed. this suggests that at some point in the past all galaxies must have been at the same point.

What is CMBR

cosmic microwave background radiation, the left over thermal radiation emitted during the early stages of the universe, it is observed today as microwave wavelengths filling all of space. it is isotropic (uniform in all directions) and fits a perfect black body spectrum corresponding to a current temperature of 2.7K

How does CMBR evidence the Big Bang theory?

shows universe was once a single incredibly hot dense state, when the universe cooled sufficiently for matter to decouple the radiation emitted originally started as high energy gamma rays, as the universe expanded the wavelengths increased until they became low energy microwaves. the fact the radiation is incredibly uniform shows that expansion happened evenly from a singular cosmic event.

what is the relative abundance of hydrogen and helium?

the matter in the universe is distributed with incredible uniformity:

75% hydrogen

25% helium

<1% heavy elements

the 3:1 ratio of hydrogen to helium is everywhere we look in space

How does the relative abundance of hydrogen and helium evidence the Big Bang theory?

nucleur fusion in stars converts hydrogen to helium, but stars haven’t existed long enough to produce anywhere near 25% helium, therefore the helium must have already been there before stars existed.

this ratio matches the theoretical predictions of rapid nuclear fusion occurring under the extreme temperature and density conditions of the early universe. as the universe rapidly expanded it cooled down below the temperature needed for nuclear fusion within a few minutes, leaving the ratio frozen at 3:1

What is the distance ladder?

no single method can measure all distances, moving from closest to furthest away:

rung 1- trigonometric parallax, astronomers measure how much a nearby star appears to shift against a background of a much more distant star as the earth orbits the sun over six months. it only works for stars that are relatively close to us, otherwise parallax angle becomes too tiny.

rung 2 - standard candles, objects of known brightness, if we know their absolute magnitude and then find its apparent magnitude you can calculate its distance using m-M=5logd/10

rung 3- measure redshifting of spectral lines from a galaxy to find its recession velocity, then use v=Hd

What is the Hubble Tension problem?

Where the two main ways we have of measuring the hubble constant give two completely different answers.

the early universe method- scientists look at CMBR and use standard model of physics to project forward and calculate how fast the universe should be expanding today. H=67.4

the late universe method- type 1 a supernovae are used as standard candles to determine the precise distance to distant galaxies, combined with redshifts data, H=73

the tension suggests either an undetected systematic error in the cosmic distance ladder or that our current standard model of cosmology is incomplete.

what is dark energy?

a hypothetical invisible form of energy that permeates all of space and is responsible for causing the expansion of the universe to accelerate

Why was dark energy invented?

in the late 1960s, astronomers used type 1a supernovae to measure distances to very far away galaxies, they discovered these distant supernovae were fainter than expected based on their red shift. the only explanation for them being so far away was that the expansion of the universe isnt slowing down due to gravity as was assumed but is actually speeding up. since something had to be providing the push to overcome the gravitational pull of all matter in the universe, scientists named this mystery driving force dark energy.

Why is dark energy controversial?

It cannot be detected directly, nothing is known about its nature or its origins, no mechanisms can currently explain how it drives the accelerated expansion.

What are Quasars?

an extremely distant highly luminous active galactic nucleus powered by a supermassive black hole. they are the most distant measurable objects in the known universe

what are the properties of quasars?

• extremely large redshift, proving they are moving away from us at extreme speeds and are located at immense distances

• high luminosity/power output, despite being so far away we can still clearly detect them from earth this implies they have a greater power output than an entire galaxy

• produced by active supermassive black holes, quasars are formed at the centre of galaxies, the extreme power output does not come from the black hole itself but from the accretion disc of matter surrounding it. as massive amounts of gas and dust rapidly spiral inward, huge amounts of gravitational potential energy are compressed and transferred into em radiation.

• strong radio sources, they were discovered in 1960s as strong emitters or radio waves

What is an exoplanet?

A planet found outside our solar system in orbit around another star.

Why are exoplanets difficult to detect directly?

Light from the host star is much brighter than the reflected light from the planet, they subtend small angles compared to the resolution of telescopes.

What is the transit method of detecting exoplanets?

monitoring the apparent magnitude of a star to look for a periodic dip in brightness caused by a planet passing directly between the star and earth. the depth of the dip gives the planets radius, the time between dips gives the orbital period. requires perfect edge on orbital alignment with earth, small planets produce tiny barely detectable dips.

what is the radial velocity method to detect exoplanets?

both planet and star orbit common centre of mass, because star is much heavier, the centre of mass sits deep inside star but slightly off centre, this causes the star to have periodic motion moving towards and away from earth which changes its radial velocity along our line of sight. this periodic movement causes a Doppler shift in the wavelength of the stars spectral lines, the lines are blue shifted as the star moves towards earth and red shifted as it moves away. by measuring the period of this shift we can calculate the planets orbital period and the amplitude of the shift allows them to calculate the planets minimum mass

what are the limitations of the radial velocity method?

method only provides a minimum mass estimate for the planet, any orbital alignment apart from face on, method is heavily heavily biased towards massive planets orbiting very close to their stars.