HUBBLE CONSTANT
1. Context
About 13.8 billion years ago, a really small, really dense, and really hot spot lying beyond spacetime began to expand. Its expansion and cooling – in an event that scientists have called the Big Bang – produced the universe as we know it. The universe continued to expand, at first really rapidly before slowing down to a great degree. Then, about five or six billion years ago, dark energy – an unknown and largely uncharacterized form of energy – accelerated its expansion again. Scientists confirmed that the universe was indeed expanding at an accelerating rate in 1998.
2. Hubble Constant
- Hubble's Constant, denoted as "H0," is a fundamental cosmological parameter that represents the rate at which the universe is expanding.
- It is named after the American astronomer Edwin Hubble, who, in 1929, provided the first observational evidence for the expansion of the universe. Here are some key points about Hubble's Constant:
- Hubble's Constant is a measure of how fast galaxies are receding from each other as the universe expands. It quantifies the relationship between the velocity at which a galaxy is moving away from us (or any observer) and its distance from us.
- Edwin Hubble made groundbreaking observations of galaxies beyond our Milky Way using the redshift of their light. He noticed that the light from these galaxies was shifted towards longer wavelengths, indicating that they were moving away from us. The greater the distance, the higher the redshift.
- Hubble's Constant is encapsulated in Hubble's Law, which is expressed as v = H0 * d. Here, 'v' represents the velocity of a galaxy moving away from us, 'H0' is Hubble's Constant, and 'd' is the distance to the galaxy.
- Hubble's Constant is typically measured in units of kilometers per second per megaparsec (km/s/Mpc). It represents the rate at which the universe is expanding per unit distance.
3. Measuring the Hubble Constant
To calculate the Hubble constant, scientists need two details: the distance between the observer and astronomical objects and the velocity at which these objects are moving away due to the universe's expansion.
Three methods are used for measurement:
- Supernova Brightness: Scientists compare the observed brightness of supernovae with their expected brightness to determine their distance. They also measure the redshift, the stretching of light's wavelength, to calculate their moving-away velocity.
- Cosmic Microwave Background (CMB): Changes in the CMB, radiation from the Big Bang, are used to estimate the Hubble constant.
- Gravitational Waves: These ripples in spacetime, produced by massive objects colliding, are detected by gravitational wave detectors. The shape of the waves helps calculate the energy released in the collision, enabling the estimation of distance. Redshift is used to determine moving-away speed.
4. The Discrepancy in Hubble Constant Measurements
- Measurements from the first method indicate a Hubble constant roughly two units higher than the second method. The third method is not yet precise enough for measurement.
- The discrepancy could be attributed to methodological errors or suggest that the Hubble constant is changing over time.
- This arises because the three methods estimate the Hubble constant for today but rely on information from different cosmic eras. The CMB method relates to a younger universe, while the other two pertain to an older universe, closer to the present day.
5. Lensed Gravitational Waves
- In 1919, Arthur Eddington observed the bending of starlight by the Sun's gravitational field during a solar eclipse, a phenomenon known as gravitational lensing.
- In 2021, researchers with gravitational-wave detectors in the U.S. and Italy began searching for lensed gravitational waves in existing data, inspired by the idea that gravitational waves could also be lensed.
- Although no lensed gravitational waves have been found yet, upcoming detectors are expected to detect about a million gravitational waves annually, increasing the chances of finding them.
- Lensed gravitational waves would appear as multiple copies of the same wave with time delays in the detectors.
- Researchers, including Shasvath J. Kapadia and Souvik Jana, are exploring the possibility of using these lensed waves and their time delays to gain insights into the rate of the universe's expansion, building models to study this potential relationship.
6. Independent Probe of Cosmological Parameters with Gravitational Waves
- Professor Ajith highlights the method's strength lies in its ability to independently estimate the Hubble constant from different stages of the universe's expansion, providing valuable insights.
- Souvik Jana mentions that this method can also help determine other cosmological parameters, such as the density of matter in the universe.
- A.R. Rao, a retired astrophysicist, finds the study fascinating, as it demonstrates a novel cosmological application of gravitational waves beyond their initial detection.
- Professor Bagla acknowledges the study's potential but emphasizes the need to address the challenge of a potentially low signal-to-noise ratio in identifying the source of gravitational waves, especially since this method doesn't rely on electromagnetic wave counterparts.
- The research team is exploring additional applications of their method, including investigations related to the nature of dark matter particles, as mentioned by co-author Tejaswi Venumadhav, an assistant professor of physics at UCSB.
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For Prelims: Hubble Constant, Big Bang Theory, Supernova brightness, Cosmic Microwave Background (CMB), and Gravitational Waves.
For Mains: 1. Discuss the methods used to measure the Hubble constant, the challenges in obtaining a consensus value, and the potential implications of a changing Hubble constant on our understanding of the universe's evolution and structure. (250 words).
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Source: The Hindu
