READING THE UNIVERSE USING LIGHT RIA CHHIKARA RIA CHHIKARA How We Observe the Universe Using Light Stars, galaxies, dust clouds, exoplanets: all of these emit or reflect or absorb light, creating signals that By tracing the structure of radio wave-emitting clouds, astronomers were able to map out the entire structure of our galaxy, the Milky Way, as well as other nearby galaxies. Source: https://www.scientificamerican.com RIA CHHIKARA How different wavelengths reveal different thing about the universe 1. Radio Waves (Longest Wavelengths) What they reveal: Cold, dense clouds of gas where stars are born, and high-energy particles moving in magnetic fields (synchrotron radiation). Key uses: Mapping the structure of galaxies, identifying pulsars (spinning neutron stars), and detecting quasars and active galactic nuclei (supermassive black holes). Radio waves can pass through dust clouds, revealing areas hidden to optical telescopes. 4. Ultraviolet (UV) Light What it reveals: Extremely hot, young, and energetic stars. Key uses: Mapping areas of active star formation and studying the upper atmospheres of planets (such as detecting aurorae on Jupiter). 3. Visible Light What it reveals: Stars, glowing gas, and reflected light from planets. Key uses: Studying the surface properties of stars and galaxies, and identifying chemical elements through spectroscopy (analyzing emission/absorption lines). 2. Infrared (IR) Waves What they reveal: Heat from relatively cool objects and dust, as well as light that has been "redshifted" (stretched) by the expansion of the universe. Key uses: Piercing through dense dust clouds to see baby stars forming, detecting brown dwarfs, mapping dust between stars, and observing the earliest, most distant galaxies in the universe. 5. X-rays What they reveal: Extremely hot gas (millions of degrees) and violent, high- energy phenomena. Key uses: Probing the environment near black holes and neutron stars, studying supernova remnants, and observing hot gas in galaxy clusters. 6. Gamma Rays (Shortest Wavelengths) What they reveal: The most energetic events in the universe, such as supermassive black holes or colliding neutron stars. Key uses: Identifying gamma-ray bursts (massive explosions) and studying the most violent, high-energy, and transient events in the cosmos. Source: https://science.nasa.gov RIA CHHIKARA How do astronomers use the light we receive now to look back in time Light takes time to travel through space, so when astronomers observe distant objects, they see them as they existed in the past. This is known as lookback time. The farther away an object is, the longer its light takes to reach Earth. For example, the Sun is observed as it was 8 minutes ago, while galaxies billions of light-years away are seen as they were billions of years in the past. This allows astronomers to look back in time and study earlier stages of the universe. Lookback Time: Seeing the Past Through Light Lookback time helps astronomers understand how the universe has changed over billions of years. By comparing nearby galaxies (more recent) with distant galaxies (older), scientists can observe how galaxies form and evolve. Light from distant objects is often redshifted, meaning its wavelength stretches as the universe expands, which helps determine their distance and age. Powerful telescopes such as the James Webb Space Telescope can detect extremely distant galaxies, allowing scientists to study the early universe shortly after the Big Bang. Using Lookback Time to Study Cosmic Evolution Source: https://www.scientificamerican.com Absorption and Emission Spectra Absorption and emission are quantum processes where electrons transition between discrete energy levels by interacting with photons. Absorption occurs when an atom absorbs a photon gaining energy to excite an electron to a higher energy state. Emission occurs when an electron drops to a lower state, releasing a photon with energy equal to the difference between levels. Spectra reveal the composition of stars and galaxies by acting as a chemical fingerprint, where unique absorption or emission lines in light correspond to specific elements present in the gaseous atmosphere. By analysing these spectral "fingerprints" using spectroscopy, astronomers can identify elements, determine stellar temperatures, and calculate the density of celestial objects. RIA CHHIKARA Source: https://imagine.gsfc.nasa.gov RIA CHHIKARA Redshift, Hubble’s Law, and the Expanding Universe Redshift is the stretching of light waves from an object, shifting them towards the red end of the electromagnetic spectrum (longer wavelengths, lower frequency) as that object moves away from the observer. What is redshift Astronomers detect redshift by using spectroscopes on telescopes to analyse the light from celestial objects, creating a spectrum of coloured light with specific absorption or emission lines. How astronomers detect redshift in spectral lines Redshift is evidence that galaxies are moving away because it indicates their light waves are stretched to longer, redder wavelengths as they recede, similar to how a moving siren’s pitch drops. This doppler-like-effect shows that distant galaxies are moving away from us, with greater redshift corresponding to faster speeds, proving the universe is expanding. How is redshift evidence that galaxies are moving away RIA CHHIKARA Source: https://www.esa.int Source: https://lco.global RIA CHHIKARA How measuring redshift helps determine the distance and speed of galaxies Measuring redshift is the primary method astronomers use to determine both the distance to far-off galaxies and how fast they are moving away from us. This technique is based on the Doppler effect where light waves stretch as an object moves away and, on a larger scale, the expansion of the universe (cosmological redshift). Source: https://lco.global The Big Bang The universe began 13.8 billion years ago not as an explosion in space, but as a rapid expansion of space itself from an extremely hot, dense, and tiny state. This initial "Big Bang" event, driven by cosmic inflation, transformed a high-energy singularity into a rapidly cooling, expanding "soup" of particles that eventually formed matter, atoms, and eventually stars. Redshift and spectral analysis provide foundational evidence for the Big Bang theory by demonstrating that the universe is expanding. By analysing the light from distant galaxies (spectra), astronomers observed that this light is stretched (redshifted) as it travels through space, proving that galaxies are moving away from us and that the universe was once concentrated in a much smaller, denser state. How does redshift and spectra provide evidence for the Big Bang years ago... 13.5 B RIA CHHIKARA Source: https://www.amnh.org Source: https://science.nasa.gov CASE STUDY: 3C 273 3C 273 is a quasar, an extremely luminous active galactic nucleus, located in the Virgo constellation about 2.5 billion light-years from Earth, and was the first quasar identified in 1963 by Maarten Schmidt; it contains a supermassive black hole of approximately 1 billion solar masses at the center of a giant elliptical galaxy, appears as a 12.9 magnitude object with a visible high-energy jet, and its discovery helped solve the mystery of quasars by proving that extremely distant and energetic objects exist in the universe. The spectral lines of 3C 273 reveal the presence of elements including hydrogen, carbon, magnesium, and oxygen, which are identified by their unique emission lines that act as chemical fingerprints in its spectrum. What was found in it RIA CHHIKARA This redshift shows that 3C 273 is moving away from Earth at high speed, its distance can be calculated using Hubble’s Law (v = H₀ × d), and its light, which left about 2.5 billion years ago, provides evidence for the expanding universe and supports the Big Bang theory. The light from 3C 273 is redshifted, meaning its wavelengths are stretched toward the red end of the spectrum, which shows that it is moving away from Earth due to the expansion of the universe. What does this tells us about its motion, distance, and place in the expanding universe Is it redshifted or blue shifted? Source: https://www.britannica.com Source: https://www.astronomy.com RIA CHHIKARA RIA CHHIKARA RIA CHHIKARA Sources https://www.scientificamerican.com/article/light-is-how-astronomers-read-the- story-of-the universe/#:~:text=Stars%2C%20galaxies%2C%20dust%20clouds%2C,glow%20with% 20their%20own%20light https://science.nasa.gov/ems/07_infraredwaves/#:~:text=Many%20objects%20in%2 0the%20universe,visible%20light%20using%20optical%20telescopes https://imagine.gsfc.nasa.gov/features/cosmic/farthest_info.html#:~:text=About%2 0the%20Image,appeared%2013%20billion%20years%20ago https://imagine.gsfc.nasa.gov/science/toolbox/spectra1.html#:~:text=Each%20elem ent%20in%20the%20periodic,neutron%20star%20or%20black%20hole https://lco.global/spacebook/light/redshift/#:~:text=More%20generally%2C%20astr onomers%20use%20redshift,above%20to%20answer%20the%20following https://www.amnh.org/explore/ology/astronomy/how-did-the-universe-begin · https://science.nasa.gov/mission/hubble/science/science-behind-the- discoveries/hubble-cosmological- redshift/#:~:text=Light%20from%20galaxies%20stretches%20to,end%20of%20the% 20electromagnetic%20spectrum. · https://www.esa.int/Science_Exploration/Space_Science/What_is_red_shift · https://www.britannica.com/topic/3C-273 · https://science.nasa.gov/mission/hubble/science/science-behind-the- discoveries/hubble-cosmological- redshift/#:~:text=Light%20from%20galaxies%20stretches%20to,end%20of%20the% 20electromagnetic%20spectrum. https://www.astronomy.com/observing/101-must-see-cosmic-objects-quasar-3c- 273 THANK YOU Service RIA CHHIKARA