MPSC 2025 Prelims Paper-I · Question 54 of 100

Correct — option (1), Red shift in the light coming from distant galaxies. When a light-emitting source moves away from an observer, the wavelengths of its light stretch to longer, redder values before reaching the observer — this is the Doppler effect applied to light, and the resulting shift is called a redshift. In the late 1920s, Edwin Hubble measured the spectra of a large sample of distant galaxies and found that nearly all of them showed a redshift, and — critically — that the amount of redshift increased roughly in proportion to a galaxy's distance from us (the relationship now called Hubble's law). The only way for essentially every distant galaxy to be receding from us, with more distant ones receding faster, is if space itself is expanding uniformly, carrying galaxies apart from one another — not that our galaxy happens to sit at a special centre that everything else is fleeing. This observation is the direct empirical foundation of the expanding-universe model and, traced backward in time, of the Big Bang theory. A universe that was truly static, by contrast, would show no systematic wavelength shift correlated with distance in the light of distant galaxies.

• (2)Blue shift in the light coming from distant galaxies — A blueshift indicates a source moving toward the observer, compressing light to shorter wavelengths — this is observed only for a small number of nearby galaxies (such as the Andromeda Galaxy, which is gravitationally approaching the Milky Way, and a few other members of the Local Group), not for the general population of distant galaxies. Since the vast majority of distant galaxies show redshift rather than blueshift, and the effect grows with distance, blueshift is not the observation that established universal expansion.
• (3)Bending of light while passing near to the Black hole — Gravitational lensing — light bending as it passes a massive object such as a black hole or galaxy cluster — is a confirmed prediction of General Relativity about how mass curves spacetime, but it says nothing about whether the universe as a whole is expanding, contracting or static. It is evidence for gravitational curvature of light paths, an entirely separate phenomenon from the cosmological redshift-distance relationship that reveals expansion.
• (4)Constancy of velocity of light — The constancy of the speed of light in vacuum, regardless of the motion of the source or observer, is a foundational postulate of Special Relativity, confirmed by experiments such as the Michelson-Morley experiment. It describes a fixed property of light's propagation, not a measurement about galaxies receding from one another, so it has no bearing on whether the universe is expanding.

Cosmological redshift arises because, as light from a distant galaxy travels through expanding space to reach us, the space it is travelling through is itself stretching, which stretches the light's wavelength along with it — a subtly different mechanism from an ordinary Doppler shift caused by a source moving through static space, though it produces a directly comparable stretching effect. Edwin Hubble's 1929 observation that galaxy redshift increases roughly linearly with distance (v = H0 x d, where H0 is the Hubble constant) is the founding empirical evidence for an expanding universe, and it is this expansion, extrapolated backward in time to a single origin point, that underlies the Big Bang model of cosmic history.

MPSC's science section tests recognition of the specific observational evidence behind major physics and astronomy theories, rather than the theories' conclusions alone — knowing that 'the universe is expanding' is less useful than knowing which specific, named observation (galactic redshift, and its correlation with distance via Hubble's law) is the evidence for it, since that is the kind of precise attribution these questions are built to test.

• Cosmological redshift is the stretching of light to longer wavelengths as it travels through expanding space from a distant galaxy to an observer.
• Edwin Hubble's 1929 observations showed that the redshift of distant galaxies increases roughly in proportion to their distance, a relationship now known as Hubble's law.
• A small number of nearby galaxies, including Andromeda, show blueshift because local gravitational attraction currently outweighs the large-scale expansion of space between them and the Milky Way.
• The observed expansion of the universe, extrapolated backward in time, is the basis of the Big Bang theory of cosmic origin.

• Confusing gravitational lensing (light bending near massive objects) with cosmological redshift — both involve light and gravity or motion, but only redshift bears on universal expansion
• Assuming every galaxy shows redshift with no exceptions, when a few nearby, gravitationally bound galaxies like Andromeda show blueshift instead
• Treating the constancy of the speed of light (a Special Relativity postulate) as cosmological evidence, when it is unrelated to the expansion question

MPSC's science section frequently asks which specific observation or experiment confirms a major scientific conclusion, rewarding precise recall of named evidence (redshift and Hubble's law, for universal expansion) over general familiarity with the conclusion itself.

No directly related past PYQ was found.

• practice — not a real PYQ

  Hubble's law describes the relationship between which two quantities for distant galaxies ?

  • (a)Mass and luminosity
  • (b)Recession velocity (redshift) and distance
  • (c)Rotational speed and diameter
  • (d)Temperature and colour

  Answer(b) Recession velocity (redshift) and distance — Hubble's law states that a galaxy's recession velocity, inferred from its redshift, increases roughly in proportion to its distance from us.

• practice — not a real PYQ

  Which nearby galaxy is a well-known exception that shows a blueshift rather than a redshift, because it is gravitationally approaching the Milky Way ?

  • (a)Triangulum Galaxy
  • (b)Whirlpool Galaxy
  • (c)Andromeda Galaxy
  • (d)Sombrero Galaxy

  Answer(c) Andromeda Galaxy — it is part of the Local Group and is gravitationally bound to the Milky Way, so it approaches rather than recedes, producing a blueshift.