The Age of the Universe Flashcards
what is red shift
the shift in wavelength of a stars spectrum
what causes red shift
the increasing separation between a star and earth
what is the name given to the effect of light red shifting due to the increasing separation between a star and earth
the doppler effect
what causes the doppler effect in general
- when an observer receiving waves emitted from a moving body observes that the wavelength of the waves has been altered
- as a consequence of the objects motion
if the siren on a stationary car was on and you were standing sill, what would you hear and why
- the siren having a constant pitch
- because the sound waves are travelling in all directions
- but the car being still means that the wavelengths of them are constant around the whole car
- therefore making the frequency of them constant
if the siren on a moving car was on and the car was driving past you, what would you hear
- the pitch of the siren would increase as the car approached you
- then when it passed you the pitch would decrease
- the increased pitches would be above that than if the car was stationary
- and the decreases pitches would be below that than if the car was stationary
why does the pitch increase as the car comes towards you
- the car is driving in the same direction to the sound waves that are approaching you
- this causes the distance between the sound waves to decrease as the wavefronts are compressed
- this decrease in wavelength results in an increase in frequency (v = fY)
- resulting in a higher pitch
why does the pitch decrease as the car drives away from you
- the car is driving in the opposite direction to the sound waves that are approaching you
- this causes the distance between the sound waves to increase as the wavefronts are stretched
- this increase in wavelengths results in a decrease in frequency (v = fY)
- resulting in a lower pitch
what is the motion of the car and the change in the sirens pitch an example of
the doppler effect
how would the EM spectra of a receding star compare to its its non receding counterpart
- the spectral lines for the receding star would have shifted towards the red end of the spectrum
- telling us that the wavelength of light is longer than it would be if it wasnt travelling away from us
how would the spectral lines of an approaching star compare
- its spectral lines would shift towards the blue end of the spectrum
- because its wavelengths of light would have decreased due to the doppler effect
what is this shift towards the shorter wavelengths of light called
blue shift / anti-red shift
how would the EM spectra of a star closer to us and a star further away from us both receding at the same speed compare (doesnt actually happen but for understanding’s sake)
- they would essentially have the same amount of red shift in their line spectra
- because the distance isnt what determines the intensity of the doppler effect
- but the speed at which the object is moving towards or away from the observer
therefore, what does the amount of red shift a galaxy exhibits allow us to calculate about it
the speed at which it is moving
what equation allows to calculate this speed
z = (delta Y / Y) = (delta f / f) = (v / c)
what do each of those variables stand for
- z = red shift ratio
- Y = lambda (m)
- f = frequency (Hz)
- v = velocity of star / galaxy
- c = speed of light (3x10^8)
in a lab sample the hydrogen alpha spectral absorption line has wavelength 656.285nm. in the spectrum for a nearby star this line is absorbed at a wavelength of 656.315nm. is this star moving towards or away from us and why
- it is moving away from us
- because there has been an increase in its wavelength detected from the star (656.315 - 656.285 = +0.03nm)
- an increase in wavelength means red shift has occurred
how would you calculate how fast the star is moving away from us
- we have the wavelengths and the change in wavelength
- so use z = (delta Y / Y) = (v / c)
- delta Y = 0.03nm and Y = 656.285nm (the normal wavelength)
- so 0.03 / 656.285 = v / 3x10^8
- rearrange for v = (0.03 * (3x10^8)) / 656.285
- v = 13700 ms-1
why dont you need to convert the wavelengths into metres
because you are calculating ratios so it doesnt matter
what is the practical relationship between a galaxys red shift and the distance it is away from us
- they are directly proportional
- an increase in one means implies an increase in the other
why is this relationship proportional if there is no direct link between the distance a galaxy is from us and its red shift
- because the further away a galaxy is
- the faster that galaxy is moving
- and therefore the more red shift there is
why does an increase in distance mean the galaxy is moving further away at a quicker speed
- this is due to the expansion of the universe
- so for a certain amount of time, a galaxy closer to us wouldve had less space ‘created’ between it and us than a further away galaxy would have had
what is the typical model used to explain this phenomenon
- a balloon with dots on it
- when blown up, the increase in the distance between two far away dots would be larger than two closer dots
how did hubble come to the conclusion that the universe was expanding using different galaxies line spectra
- he discovered that basically all galaxies showed red shift in their spectra
- meaning that all galaxies were moving away from us (and each other)
- therefore the universe had to be expanding
what is hubbles law
v = H(0) * d
what do each of those variables mean
- v = velocity of recession
- H(0) = hubbles constant
- d = distance between body and earth (in Mpc)
what is the value of hubbles constant
H(0) = 70.9 kms-1Mpc-1 or 70,900 ms-1…
a supernova appears in the sky and astronomers find that it has a red shift of z = 0.45. what is the speed of the supernova
- z = v / c
- v = z c
- v = 0.45 x (3x10^8) = 1.35x10^8 ms-1
how far away is the supernova
- v = 1.35x10^5 kms-1 for similar units
- using v = H(0) d, d = v / H
- (1.35x10^5) / 70.9 = 1900Mpc
- d = 1900 Mpc
regarding z, how do you know if your shift is red or blue and why
- red shift = +ve delta Y
- blue shift = -ve delta Y
- because change in wavelength is calculated by subtracting the new value by the old one
- so red shift which has a new value bigger than the old one gives a +ve value
what does pc stand for again
parsec
what is the distance one 1 pc
3.09x10^16 m
therefore what is the distance of 1 Mpc
- mega = 1x10^6
- so (3.09x10^16) x (1x10^6) = (3.09x10^22 m)
given d = 1900 Mpc, what is that distance in meters
1900 x (3.09x10^22) = 5.871x10^25 m
why is finding more accurate values of the hubble constant so important
- because it will tell us how quickly the universe is expanding
- from this we can accurately work out how long ago the big bang was
- and when (if it will) our universe dies
for an object to travel a distance d(0) from the beginning of time, at a speed pf v(0), the time T(0) could be calculated using what basic equation
time = distance / speed
given T(0) = d(0) / v(0), what relationship does the hubble constant and time have
- as v = H(0) * d, H(0) = v(0) / d(0)
- given T(0) = d(0) / v(0)
- T(0) = 1 / H(0)
how was this equation used to calculate the age of the universe
- we know H(0) = 70.9 kms-1 Mpc-1 = 70,900 ms-1 Mpc-1
- and 1 Mpc = 3.09x10^22 m
- therefore 70,900 / 3.09x10^22 = 2.29x10^-18 ms-1 m-1
- the m and m-1 cancel out
- giving H(0) = 2.29x10-18 s-1
- using T(0) = 1 / H(0), diving 1 by 2.29x10-18 s-1 (where 1/1 divided by 1/s = 1/1 *s/1 = s) gives us the unit we want
- 1 / 2.29x10-18 s-1 = 4.36x10^17s
- which tells us the universe is roughly 13.8 billion years old
how do you get the step where you divide 70,900 by 1 Mpc, surely you should be multiplying to cancel out the Mpc-1 in the units right?
- 70900 ms-1 Mpc-1 is the same as 70900 (m / s Mpc)
- therefore inputting the value of 1 Mpc (as the unit equates to ‘per’ Mpc) into the units gives you a division
- this replaces the Mpc-1 unit with the m-1 unit
what is the big bang
- the theory that he universe expanded outwards
- from a single point to the currently observed situation
