Comment Mesurer L'Univers ? 🔭🌕☀️✨🌌

Summary

This script delves into the fascinating methods astronomers use to measure cosmic distances. Starting with the laser telemetry for the Moon, it explains the historical parallax method, used since Hipparchus, and its modern application with the Earth-Sun distance as a baseline. The script explores the transit of Venus and radar telemetry for solar system measurements. It then describes using parallax with Earth's orbit to measure nearby stars and introduces 'standard candles' like Cepheid variables and Type 1a supernovae for more distant galaxies. Finally, it discusses the Hubble Law for the most distant galaxies and the current crisis in cosmology regarding the Hubble constant's value.

Takeaways

• 📏 The script discusses various methods used in astrophysics to measure astronomical distances, such as light-years for stars and larger scales for galaxies.
• 🌌 It explains the concept of the cosmic distance ladder, a series of methods that rely on one another to measure distances in the universe.
• 🌕 The distance to the Moon is measured using laser telemetry, with reflectors placed by lunar missions to bounce back the laser light, allowing for precise distance calculations.
• 🔭 The ancient method of parallax, first used by Hipparchus, involves observing an object's apparent shift against a distant background from different viewpoints to calculate its distance.
• 🌟 The script describes how the parallax method is applied to stars, using the Earth's orbit around the Sun as a baseline to measure tiny angles of parallax, which can be translated into distances.
• 🛰️ The European Space Agency's Hipparcos satellite and its successor, the Gaia satellite, have been used to measure parallaxes of stars with high precision, creating extensive star catalogues.
• 💡 The concept of 'standard candles' in astronomy is introduced, which are astronomical objects with known intrinsic luminosity, allowing for distance estimation based on their apparent brightness.
• 🌌 Henrietta Leavitt's work with Cepheid variable stars provided a way to use these stars as standard candles, with their period of luminosity variation being directly related to their intrinsic brightness.
• 💥 Supernovae, particularly Type 1a, are used as another type of standard candle due to their consistent peak brightness, allowing for the measurement of very distant galaxies.
• 🌌 Hubble's Law, which relates the velocity at which galaxies recede from us to their distance, is calibrated using supernovae and Cepheid variables, enabling the estimation of distances across the observable universe.
• 🔍 The script concludes with a current debate in cosmology regarding the discrepancy between different measurements of the Hubble constant, which could have significant implications for our understanding of the universe's expansion rate.

Q & A

How are distances measured in astrophysics when direct measurement is not possible?

In astrophysics, distances are measured using a series of methods that form what is known as the cosmic distance ladder. These methods are interdependent, with each one being calibrated using the previous one, much like climbing a ladder.

What is the method used to measure the distance to the Moon?

The distance to the Moon is measured using laser telemetry. A laser is fired from Earth, and the time it takes for the light to travel to the Moon and back is measured. Reflectors left on the Moon by lunar missions help in reflecting the light back to Earth.

How does the parallax method work for measuring distances to celestial objects?

The parallax method is based on the geometric principle where the apparent position of an object changes when viewed from different positions. By observing the angle of this apparent shift, astronomers can use trigonometry to calculate the distance to the object, such as the Moon or stars.

What is the significance of the third law of Kepler in determining the distance to the Sun?

Kepler's third law states that the cube of the distance of a planet from a star is proportional to the square of its orbital period. This relationship allows astronomers to calculate the distance between the Earth and the Sun by comparing the orbital periods of Earth and other planets like Venus.

How did Edmond Halley's method contribute to measuring the distance to the Sun?

Edmond Halley proposed using the transit of Venus across the Sun to measure the Earth-Venus distance. By observing the transit from different points on Earth, astronomers could apply the parallax method to find the distance between Earth and Venus, which, combined with Kepler's third law, helped estimate the Earth-Sun distance.

What is the concept of 'standard candles' in astronomy?

In astronomy, 'standard candles' are objects with known intrinsic luminosity. By comparing their apparent luminosity to the known intrinsic luminosity, astronomers can estimate their distance. This concept is crucial for measuring distances to objects beyond our galaxy.

What role did Henrietta Leavitt play in measuring cosmic distances?

Henrietta Leavitt discovered the period-luminosity relationship for Cepheid variable stars. This relationship allows astronomers to determine the intrinsic luminosity of Cepheids, making them useful as standard candles to measure distances in the universe.

How are distances measured to stars using parallax and the Earth's orbit around the Sun?

By observing a star at six-month intervals, astronomers can measure its parallax using the Earth's orbit around the Sun as a baseline. This method allows for the calculation of distances to stars within our galaxy.

What is the significance of the Hubble constant in cosmology?

The Hubble constant, denoted as Ho, is the coefficient of proportionality in Hubble's law, which relates the recessional velocity of galaxies to their distance. It is crucial for estimating distances to galaxies across the observable universe.

How does the method of using supernovae as standard candles work?

Type 1a supernovae have a consistent peak luminosity due to their uniform explosion mechanism. By comparing the observed brightness of these supernovae to their known peak luminosity, astronomers can measure distances to galaxies where they occur.

What is the current crisis in cosmology regarding the Hubble constant?

There is a discrepancy between the values of the Hubble constant measured using different methods. While one method suggests a value of around 73 km/s/Mpc, another based on cosmic microwave background radiation suggests 67. This tension indicates a potential issue with one or both of the measurements.