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By 1929, Hubble had completely reimagined our place in the universe; not only was it home to millions of other galaxies, but the universe itself was expanding as well. Contrary to the previously held view of a static universe, Hubble ultimately proved that galaxies are, in fact, moving away from us. By studying the light emitted from various galaxies, Hubble discovered that the light appeared displaced toward the red end of the spectrum. It became apparent that our universe was ceaselessly expanding outward, and all galaxies housed within it were moving away from one another. This phenomenon, known as redshift, reveals that the farther a galaxy is away from us, the redder its light will appear. Hubble also demonstrated that galaxies farther away from us are receding faster than those nearby – a fundamental observation now known as Hubble’s Law. The idea of an expanding universe is a key underpinning of the Big Bang Theory. Hubble’s observations provided the earliest insight into the origins of our universe.
Left: A photographic plate of the Andromeda galaxy, upon which Edwin Hubble first noted a novae, then crossed that out and added "Var!" when he discovered the star was in fact a cepheid variable. Right: The Hubble Space Telescope revisited Hubble's famous cepheid variable star V1 between December 2010 and January 2011.
Left: Carnegie Observatories. Right: NASA, ESA, and the Hubble Heritage Team (STScI/AURA); Acknowledgment: R. Gendler
Red Shift - The Doppler Effect on a Galactic Scale
Red Shift - The Doppler Effect on a Galactic Scale
Compared to laboratory reference spectra, light from distant stars and galaxies are shifted to either the longer end of the spectrum (Red Shift) or the shorter end of the spectrum (Blue Shift).
Astronomers have measured the change in wavelength (Δλ) most cases to be shifted to red portion of the spectrum. While it is referred to Red Shift, the shift applies to all wavelengths across the EM spectrum.
The Red Shift value is z. as determined by the equation to the right. The initial wavelegth (λ0 ) and the Δλ = λ - λ0 . It is a ratio so therefore it is unitless.
Key Differences:
Cosmological redshift: The space between the source and the observer has expanded between the time when the radiation was emitted and the time when it was received.
Doppler effect: The source of radiation and the observer are moving relative to each other (disregarding the expansion of space).
Red Shift Sample Problems
Red Shift Sample Problems
Suppose you observe a galaxy for which the bluegreen (486 nm) line of hydrogen is red shifted to the red end of the visible spectrum at 700 nm. What is the recession speed of the galaxy? Ans: z = 0.440, v = 1.32 x 108 m/s
One of the visible hydrogen spectral lines, the aqua H-β line, has a laboratory wavelength of 486.1 nm. In a spectrum of a distant galaxy, this line is observed to have a wavelength of 535 nm. What is the red shift and recessional velocity of this galaxy? Ans: z = 0.10; v ≈ 0.10c = 3.0×107 m s−1
Find the change in the wavelength of the light from a galaxy that is moving away from the Earth at 2000 kms-1. Assume that the emitted wavelength is 600 nm. Ans: Δλ = 4 nm
Scientists can use the visible light spectrum from distant stars to determine whether the stars are moving. The visible light spectrum from stars includes dark lines at specific wavelengths. The diagram shows the visible light spectrum from the Sun and from four other stars, A, B, C and D.
Which star, A, B, C or D, is moving away from the Earth? Ans: C
How does the speed of star B compare with the speed of star D?
The speed of star B is greater than the speed of star D.
The speed of star B is less than the speed of star D. ✓
The speed of star B is the same as the speed of star D.
State one conclusion that can be drawn about star A.
5. Scientists have observed that the wavelengths of the light given out from galaxies that are moving away from the Earth are longer than expected.
a. The positions of three galaxies, A, B and C, are marked on the graph. From which galaxy, A, B or C, would the wavelength of the light reaching the Earth seem to have changed the most? Ans: C
b. Give a reason for your answer.
Hubble's Law
Hubble's Law
Measuring the distance to many Cepheid Variable stars, known as 'standard candles or standard candles' , and determining their rate of recession using his redshift calculations, Hubble was able to create the plot the following data. He then derived the following:
Hubble's Law:
the current velocity of recession, v, of a galaxy is proportional to its distance from the Earth, d.
From the plotted graph, determine Hubbles constant (H0 ). Ans: ~75 km s-1 /Mpc
As of 2016, using the Hubble Space Telescope,
Using Hubble's Constant to determine Age and Size of Universe
Using Hubble's Constant to determine Age and Size of Universe
Assumptions:
Assumptions:
Constant Expansion Rate (73 km s-1 /Mpc)
Light travels at 3.00 x 108 m s-1
Light is from the edge of the observable universe.
Age of Universe:
Age of Universe:
Using the above value, determine the age of the universe in years (Earth years).
Hints:
Convert Mpc into 3.08x1022 m
Convert km s-1 in m s-1
Solve for seconds
Ans: T = 4.3 x 1017 s = 1.4 x 1010 y
Size of Universe:
Size of Universe:
Determine the size of the observable universe in meters.
Ans: d = 4.4 x 1026 m.
Cosmic Scale Factor
Cosmic Scale Factor
The Cosmic Scale Factor represents how the universe has changed in size over time. In the figure to the right, the 'before' time is 0.55 of 'now'. Likewise, 'later' is 1.45 of 'now'.
The pattern of the stars is the same, it has 'simply' expanded in its scale.
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