- how the expansion of the Universe was found, - to describe the expansion of the Universe.
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Before you start, try to answer the questions.
What is a galaxy?
What types of galaxies are known?
What does a galaxy consist of?
Redshift phenomenon
Definition: Redshift phenomenon
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RedshiftredshiftRedshift describes how the observed light spectra are shifted towards longer wavelengths (red part of visible light spectrum) as an object in space moves further away from the observer.
This phenomenonphenomenonphenomenon allowed to find out that the Universe expands.
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The redshiftredshiftredshift of an object is measured by studying the absorption or emission lines in its spectrum. The spectrum lines are unique for every element and correspond always to the same wavelengths (frequencies). When an object moves in space toward or away from its observer, the spectrum lines can be found in different position, as they were expected.
If the space between galaxies expands, the wavelength of the light sent from one galaxy stretches proportionally to the time that it needs to reach the other galaxy. RedshiftredshiftRedshift (z) is defined as the change in the wavelength of the observed light divided by the wavelength that the light would have if the source was not moving (emitted wavelength):
where: ν - is the radial outward velocity of the galaxy (ν << c), c - is the speed of light.
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Hubble’s law
Definition: Hubble’s law
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In 1929 the American astronomer Edwin Hubble described the redshiftredshiftredshiftphenomenonphenomenonphenomenon and connected it to the expansionexpansionexpansion of the Universe. The results of his observations showed that nearly all galaxies are moving away from each other. The redshift appeared to be larger for faint and, as he assumed, further galaxies. Hence, the further a galaxy, the faster it is receding from the observer.
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The Hubble’s law describes the relation between the velocity of a receding galaxy and its distance to the observer as:
where: ν - is the radial outward velocity of the galaxy, r - is the distance between the galaxy and the observer, HIndeks dolny 00 - is the proportionality constant (called Hubble constant).
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The exact value of the Hubble constant is still under study. The approximate value is:
The latest value in 2018 from Planck Mission results is .
Parsec
Definition: Parsec
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A parsec is a unit often used by astronomers to measure large distances to astronomical objects outside the Solar System. One parsec corresponds to the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond. 1 parsec is about 3,3 light‑years. The standard abbreviation is pc.
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The value of the Hubble constant is important to cosmologists because it can be used to estimate the age of the Universe. If we assume that the expansionexpansionexpansion of the Universe started immediately with the beginning of its existence and it continues, then the radial outward velocity of our Galaxy can be expressed as:
If we compare these formula with the Hubble law than the lifetimelifetimelifetime of the Universe evaluated with the Hubble constant is about 13,8 billion years.
Remember
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In 1929 the American astronomer Edwin Hubble described the redshiftredshiftredshiftphenomenonphenomenonphenomenon and discovered that the Universe expands.
Exercises
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Exercise 1
Exercise 2
In an absorption spectrum of a particular galaxy a line expected to have the wavelength of 500 nm was found at 505 nm. Calculate the velocity of this galaxy. What is its distance from the Earth?
Exercise 3
Describe in English what facts are speaking for the expansionexpansionexpansion of the Universe.