Measuring the Universe

Question 1

What is parallax?

Answer 1

Parallax is used by astronomers to measure distances to nearby stars. Angular shift in radians is the ratio of the baseline to the distance from the Sun to the stars. Parallax is very small.

Question 2

What are standard candles?

Answer 2

Closer stars appear larger and brighter.
True brightness is absolute luminosity.
A standard candle is a star with known absolute luminosity. We calculate distance to it by apparent brightness - a dim star with high absolute luminosity must be more distant than the same star which is brighter.

Question 3

What are Cepheids an what do they show?

Answer 3

A variable star is a standard candle. Brightness changes over time. Henrietta Swan Leavitt found Cepheids showed brightness variation with a time period. Larger absolute luminosity typically meant a longer time period.
Relative luminosity, (absolute luminosity/luminosity of Sun), against time, shows a straight line and positive correlation..

Question 4

What observation did Hubble make in 1923?

Answer 4

In 1923 Edwin Hubble observed a Cepeid in the Andromeda Nebula. Some thought it was a gas cloud in our galaxy but the patch was at least 900,000 light years distant. It must be a galaxy.

Question 5

What is Doppler Shift?

Answer 5

Atoms absorb and emit light at particular wavelengths.
If the star is receding, wavelengths of the spectral lines rise. Receding stars show red-shift, the change in wavelength is Doppler shift.

Question 6

What is the Doppler Shift formula?

Answer 6

A light source not moving has wavelength=velocity*T. If the light source moves away at speed v, change in wavelength is vt. Change in wavelength over wavelength = v/c.

Question 7

How did Hubble obtain his equation?

Answer 7

In 1929 Hubble used recession speeds for 24 galaxies and plotted these against distance to find that further away galaxies move away faster. He found recession velocity = H0r, where H0= Hubble’s constant.
1/H0 equals time since the galaxies were close. The bigger H0, the faster the Universe is expanding, and the younger it must be.

Question 8

What is cosmological redshift?

Answer 8

Very large red shifts are caused by space expanding in distant galaxies. The wavelength of light stretches with space. This is cosmological redshift. Redshift, z= change in wavelength/wavelength. Space stretches by a factor of 1+z. When we look at objects of z=10, we look at time when the Universe was 1/10th of its current size.

Question 9

What was there in the beginning, (BBT)?

Answer 9

In the beginning there was a Big Bang. The Universe was filled with a plasma of elementary particles exchanging energy with EM radiation photons.

Question 10

What was there 300,000 years after the BB?

Answer 10

Temperature=3000K. Wavelength of radiation= 1 micrometer.
Temperature falls and atoms form as electrons are held in orbit around nuclei. The Universe becomes transparent to protons which no longer interact with atoms.
Then, the interstellar space is filled with a photon gas where temperature is proportional to energy and energy is proportional to frequency.

Question 11

What is there now in the Universe?

Answer 11

14 billion years after the Big Bang…
T=2.7K, wavelength = 1 millimetre. The Universe has expanded, stretching the wavelength of photons. As wavelength rises, frequency falls and so temperature does.

Question 12

How was cosmological red shift found?

Answer 12

In 1965 a microwave antenna was calibrated for use of satelittes. They found noise in the signal at microwave wavelengths. The radiation was cosmic background radiation from the Universe’s beginning. It has the biggest cosmological red shift and was produced when the Universe was cool enough for electrons and ions to combine to produce neutral atoms, emitting photons. This happened when the temperature was 3000K. Today the wavelength is 1000x larger.

Question 13

What is the formula to show how much the Universe has expanded?

Answer 13

Z+1 = Radius of universe now/radius of universe then, the red shift factor.

Question 14

What does cosmological red shift prove?

Answer 14

Cosmological red shift provides evidence the Universe began in a hot state, alongside Hubble’s Law. Satelittes have produced images showing radiation wasn’t smooth with temperature and pressure differences. This variation must have begun with early formation of the Universe. Stars and galaxies formed here.

Question 15

What is at rest?

Answer 15

At rest : not moving relative to things around you.

Question 16

There are only ________ velocities

Answer 16

There are only relative velocities.

Question 17

What time does a clock measure? How about an observer?

Answer 17

A clock at rest moves relative to you, and measures wristwatch time T. An observer measures time t.

Question 18

What are Einstein’s laws?

Answer 18

First law of relativity: Physical laws are the same for all observers and there is only relative velocity.
Second law: The speed of light, c, is constant no matter how it is observed.

Question 19

What are space time diagrams?

Answer 19

A spacetime diagram has y axis of time in seconds, x axis of light-seconds, distance, x/c.
Worldlines represent the paths of objects through space and time.
Sloping worldlines are for objects moving at a constant v relative to you.
Light pulses travel at 45 degrees as they travel 1 light second per second.

Question 20

What is the formula for time dilation, and what is time dilation?

Answer 20

Clocks moving relative to an observer run slowly. Each second is dilated. The greater relative velocity, the greater the time dilation. Time for one tick is 2T. For an outside observer, t= gamma * T where gamma is 1 divided by the square root of 1-v^2/c^2, and T is the time measured by the person being observed.

Question 21

What are applications of time dilation?

Answer 21

Synchronised clocks measure different times if one travels a round trip on a plane.
GPS has to account for relativistic time dilation to pinpoint locations.
Half life of particles increases when accelerated to relativistic speeds.
For photons, no time passes as the relativistic factor is infinite.