Which of the following confirms that the universe is not static but expanding ?

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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.

Why the other options are wrong — (b) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2025 Q54 — Science & Technology PYQ with Answer

State PCS 2025 Q54 — Science & Technology PYQ with Answer