It is not a joke to measure distances in universe. Reason is simple, universe is vast and objects in the universe are usually too far away. Universe is so vast that large part of it is still unobservable. Measuring distances in unobservable universe is out of question and distances can be measured in observable universe only. You will need cosmic distance ladder to measure distances in the universe.
What is cosmic distance ladder?
A ladder consists of succession of steps. Same is the case with cosmic distance ladder. It is the succession of methods used by astronomers to measure distances in the universe. It is not possible to measure all astronomical distances using one method and hence with time ladder analogy developed. Method of direct measurement is at the base of the cosmic distance ladder. Distance of only close objects can be measured directly. Methods of measuring farther objects lie higher on the cosmic distance ladder.
Methods used to measure distances in universe
Direct measurement – Use of radar and parallax to measure distances in universe fall in the category of direct measurement. Radar method of measuring distances of celestial objects involves calculation of time taken by light to reach celestial objects. Velocity of light (300000 km/sec) is known. This multiplied by time taken by light to reach a particular celestial object gives distance of that object. Distance of earth from most planets has been measured using this method. Parallax method is used to measure distance of stars closer than 100 light years.
Standard candles – Parallax method fails if distance of objects in galaxies other than our galaxy is needed. Another method that uses standard candles is useful in such a scenario. Standard candles are astronomical objects whose absolute magnitude is known. Most commonly used standard candles are Cepheid variable stars and RR Lyrae variable stars.
Hubble’s law – For very far away objects none of the aforementioned methods work and astronomers have to rely on theory propounded by Hubble. Hubble claimed in 1929 that universe is expanding. This is known as Hubble’s law.
New method of calculating distances in the universe – A new method of measuring distances using a specific subset of RR Lyrae stars is making news nowadays. RR Lyrae stars used in this method are double-mode RR Lyrae stars. It has been claimed that double-mode RR Lyrae stars are great distance and metallicity indicators.
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