Our universe has been on a vast journey of evolution – starting from a tiny, dense, hot spot beyond spacetime around 13.8 billion years ago. This spot expanded and cooled in an event known as the Big Bang, forming the universe as we know it today. The universe’s expansion began rapidly slow down until about five or six billion years ago when dark energy, a largely unknown form of energy, accelerated its expansion again. Scientists constantly grapple with understanding this expansion, particularly determining the exact rate, known as the Hubble constant, which remains a contentious issue.
Understanding the Hubble Constant
In 1929, Edwin Hubble formulated Hubble’s law, which was the first mathematical description of the universe’s expansion. The rate of this expansion is termed as the Hubble constant. Two details are required to calculate its value: The distance between the observer and astronomical objects, and the velocity at which these objects are moving away from the observer due to the universe’s expansion.
Methods of Determining the Hubble Constant
Scientists have been using three methods to gather these details. First, they compare the observed brightness of a stellar explosion, called a supernova, with its expected brightness to figure how far away it could be. Then, they measure the redshift to figure how much the light’s wavelength from the star has been stretched by the universe’s expansion.
The second method involves changes to the Cosmic Microwave Background (CMB), the radiation leftover from the Big Bang event, to estimate the Hubble constant. The CMB is referred to as the ”afterglow” of the Big Bang, filling the observable universe with a faint, nearly uniform glow of microwave radiation.
The third method uses gravitational waves, ripples in spacetime produced when massive astronomical objects, like neutron stars or black holes, collide. By comparing the amount of released energy during the collision to the energy the waves had when they reached earth, researchers estimate the distance between these objects and earth.
Discrepancy in Measurement
A discrepancy has been observed in measurements using these methods. The first method reports a Hubble constant about two units higher than the one derived by the second method; the third method is not yet mature enough to provide a precise measurement. This variance could indicate a mistake in the methods used or suggest that the Hubble constant is evolving with time.
The Novel Approach for Estimation of Hubble Constant
Recently, researchers proposed a new method to determine the Hubble constant. They suggested analyzing a collection of lensed gravitational waves and their time delays to gather information on the universe’s rate of expansion. Gravitational lensing, a phenomenon where the gravitational field of a massive object bends and distorts the light from objects located behind it, forms the basis of this approach. This method provides an independent estimation of the Hubble constant and can help determine other cosmological parameters, such as matter density.
Evidence for Continued Expansion of Universe
The continued expansion of the universe has been evidenced by discoveries like the Cosmic Microwave Background Radiation found by Arno Penzias and Robert Wilson in 1963, and redshifts of distant galaxies measured by Edwin Hubble in 1929. Both discoveries provided evidence for the expanding universe, further supporting the Big Bang theory. However, the movement of asteroids in space and occurrence of supernova explosions, while contributing valuable information about early universe material and distribution of elements throughout the universe, have not provided direct evidence for the universe’s expansion.