Indirect evidence builds, yet the ‘dark’ universe remains murky

June 17, 2024
Posted by: OptimizeIAS Team
Category: DPN Topics

Indirect evidence builds, yet the ‘dark’ universe remains murky

Sub: Science and tech

General Theory of Relativity and Dark Universe

General Theory of Relativity:

The general theory of relativity has excelled in explaining gravity and phenomena like gravitational waves, lensing, redshift, black holes, and time dilation, refining Newton’s laws by describing gravity as a geometric property of spacetime.
However, on a cosmic scale, space appears filled with dark energy, constituting about 70% of the universe’s energy, causing the universe to expand.
This contrasts with Newtonian gravity, which pulls objects together.

Important terms:

Gravitational Waves	These are waves of the intensity of gravity that are generated by the accelerated masses of binary stars and other motions of gravitating masses, and propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as the gravitational equivalent of electromagnetic waves. Gravitational waves are sometimes called gravity waves, but gravity waves typically refer to displacement waves in fluids. In 1916 Albert Einstein demonstrated that gravitational waves result from his general theory of relativity as ripples in spacetime.
Gravitational lensing	A gravitational lens is matter, such as a cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein’s general theory of relativity.
Redshift	Scientists measure cosmic distances via redshift, the extent to which light is shifted towards the red (lower energy) part of the electromagnetic spectrum during its long journey across the universe. The greater the distance, the higher the redshift.
Black hole	Black holes are formed after the supernova explosion when the core of a massive dying star more than 2.5 times the mass of the Sun collapses until an infinite density where gravity is so high that even light cannot escape. Predicted by Einstein, through his general relativity theory which says that a sufficiently compact mass can bend spacetime and create a black hole, its theoretical structure was prepared independently by Tolman, Oppenheimer, Volkoff and S. Chandrasekhar.
Time Dilation	Time dilation is the difference in elapsed time as measured by two clocks, either because of a relative velocity between them (special relativity), or a difference in gravitational potential between their locations (general relativity). When unspecified, “time dilation” usually refers to the effect due to velocity.

Dark Energy:

Observed at cosmic scales, dark energy permeates space, causing the universe’s expansion.
Comprises 70% of the universe’s energy from the Big Bang.
Creates negative pressure, stretching spacetime and allowing stars and galaxies to drift apart.
Dominates in empty space, whereas gravity prevails in matter-rich regions.

Dark Matter:

Proposed based on cosmological observations; first indirect evidence published by Vera Rubin 44 years ago.
Explains unexpected galactic rotation rates, suggesting invisible matter exerts additional gravitational force.
Remains hypothetical; no direct evidence yet.

Scientific Debate:

Not all scientists agree with dark matter and dark energy hypotheses.
Some propose alternate gravity paradigms, but they don’t fully explain observed phenomena like dark matter and dark energy do.

DESI and Lambda-CDM Model:

Broadband Search for Dark Photon Dark Matter (BREAD) search and the Dark Energy Spectroscopic Instrument (DESI) aims to create a detailed 3D map of the universe.
- By mapping the position of thousands of galaxies over many years, we can keep measuring how much the universe’s expansion due to dark energy is accelerating.
- But for now, we have no choice but to draw all our inferences about dark matter and dark energy from indirect evidence alone.
Data supports the Λ-CDM model, with Λ (lambda) representing dark energy’s density.
- Λ (lambda) is the cosmological constant: it represents the energy density of space and is closely associated with dark energy.
- It appears in equations of the general theory of relativity.
Some studies suggest dark energy might change over time, challenging the Λ-CDM model.
Modified theories like MOND, which don’t require dark energy, face challenges, such as data from the Cassini mission not supporting its predictions.

Source: TH