Why the Universe Is Expanding Faster Than Light, What Hubble and Cosmology Reveal

Albert Einstein's special relativity sets a firm ceiling: nothing with mass can accelerate through space faster than light. Yet galaxies beyond roughly 14 billion light-years from Earth are already receding from us at speeds greater than 300,000 kilometres per second. No law is being violated. The distinction is precise and consequential. Special relativity governs objects moving through space. The expansion of the universe is space itself stretching. The galaxies are not racing outward like shrapnel from an explosion. The fabric they sit in is inflating, and they are carried along for the ride.
This is not a metaphor physicists use to make the concept friendlier. It is the actual geometry described by Einstein's general relativity and encoded in the Friedmann equations, which govern how a universe filled with matter and energy evolves over time. Edwin Hubble observed in 1929 that distant galaxies showed a redshift proportional to their distance. The light had not changed colour in transit because the source was moving away. It changed because the space the light travelled through had stretched, pulling the wavelengths longer, toward the red end of the spectrum. That is redshift in its cosmological form, and it is the primary tool astronomers use to measure how fast the universe is expanding.The rate of expansion is expressed as the Hubble constant, denoted H₀, measured in kilometres per second per megaparsec. One megaparsec is about 3.26 million light-years. The Hubble constant tells you how fast a galaxy recedes for every megaparsec of distance between it and us.
Two methods of measuring H₀ keep producing different answers, and the gap has refused to close as instruments have improved. Measurements based on the cosmic microwave background radiation, the afterglow of the early universe, studied in extraordinary detail by the Planck satellite, give a value of approximately 67 km/s/Mpc. Measurements using Type Ia supernovae as standard candles, a method refined by the SH0ES team led by Nobel laureate Adam Riess, consistently return values closer to 73 km/s/Mpc. Both methods are sound. Both have been checked repeatedly. The discrepancy is called the Hubble tension, and it has not been resolved. Some cosmologists think the standard model of cosmology is missing a component. Others suspect a systematic error in one of the measurement chains that has not yet been identified. The James Webb Space Telescope is now providing data that could sharpen or dissolve the tension.Redshift is often described loosely as a Doppler effect, the same phenomenon that makes a passing train's horn drop in pitch. For nearby galaxies moving through space relative to us, that description is approximately correct. For the distant universe, it is not. Cosmological redshift measures the total amount by which the universe has expanded between the moment light was emitted and the moment it arrives at a detector. A galaxy with a redshift of z = 1 emitted its light when the universe was half its current size. A galaxy at z = 7 emitted light when the universe was roughly one-eighth its current size. The most distant objects detected so far, including galaxies found by the James Webb Space Telescope at redshifts above z = 13, were emitting light less than 300 million years after the Big Bang.Because space expands, there is a distance beyond which light emitted today will never reach us. That boundary is called the cosmological event horizon. It sits at roughly 16 billion light-years. Anything beyond it is, for all practical purposes of observation, gone. Not hidden behind a barrier. Simply in a region where the expansion of space will always outpace the light trying to cross it.
The observable universe, the sphere of space from which light has had time to reach us since the Big Bang, has a radius of about 46 billion light-years. That number is larger than 13.8 billion light-years, the age of the universe in light-travel time, precisely because space has been expanding throughout the journey. The light we detect from the cosmic microwave background was emitted when the universe was about 380,000 years old. The matter that emitted it is now roughly 46 billion light-years away.

India's own contribution to probing these distances is growing. ISRO's Aditya-L1 mission, launched from Sriharikota in 2023, studies the Sun's influence on the space environment near Earth. The data feeds into a broader international picture of how local spacetime behaves, a necessary calibration layer for the cosmological measurements made by larger observatories. Gaganyaan, India's crewed spaceflight programme, will place Indian astronauts in low Earth orbit, building the human infrastructure that supports longer-range scientific ambitions. The cosmological questions are answered by satellites and ground-based telescopes, but the institutional capacity to participate in that science is built mission by mission.The expansion of the universe is not just continuing. It is speeding up. The evidence for this came in 1998 from two independent teams studying Type Ia supernovae, work that earned Saul Perlmutter, Brian Schmidt, and Adam Riess the Nobel Prize in Physics in 2011. The accelerating expansion implies the existence of something that acts as a repulsive force at cosmological scales. That something is called dark energy. It constitutes approximately 68 percent of the total energy content of the universe, according to the standard cosmological model known as Lambda-CDM. What dark energy actually is remains unknown. It may be a property of spacetime itself, a cosmological constant Einstein introduced and then famously called his greatest blunder. It may be something else entirely. The acceleration means the universe will not collapse back on itself. Distant galaxies will continue to recede, their light redshifting to longer and longer wavelengths, until they cross the event horizon and go permanently dark.
The question of why the expansion is accelerating connects directly to the Hubble tension. If the value of dark energy varied over time rather than remaining constant, that could shift the expansion rate and help explain the discrepancy between the two measurement methods. No observation has confirmed this yet. It remains one of the open problems in cosmology.

The fact that the universe is expanding faster than light in its outermost regions is not a crisis for physics. It is a clarification of what the speed of light actually limits: motion through space, not the growth of space itself. The galaxies disappearing beyond our horizon are not fleeing. They are being carried. The distinction matters because it means the observable universe is a bounded thing, not because of walls, but because of geometry, and the information it contains is finite, shrinking, and already partially gone.