Cosmic Evolution: From Scientific Method to the Big Bang
The Scientific Method and Its Application to the Cosmos
The scientific method involves making observations, asking questions, forming hypotheses, and conducting experiments. Elegance, in this context, refers to the ability of a theory to explain a wide range of phenomena.
A scientific belief is typically backed up by experiments designed to test hypotheses. If several experiments do not contradict a hypothesis, we tend to believe it to be true. While this is not directly applicable to phenomena like star formation, we still have a basis to believe related claims, even though they can be affected by metaphysics, authority, and evolved cultural beliefs. Different conclusions can be drawn from the same bodies of evidence (observations).
Careless or impartial weighting of evidence, inaccurate data due to technological limitations, background knowledge, and personal biases can lead to conflicting hypotheses.
Measuring the Universe: From Solar System to Stars
Size of the Solar System
The distance to Venus can be determined during its transit, when it moves in front of the Sun, aligning with the Earth and the Sun.
Distance to Stars
Stellar parallax can be used to measure the distance to stars. By measuring the angle of stars from observations six months apart, we can determine this distance. However, these angles are extremely small, and if stars are too far away, the angles are too small to observe. If the intrinsic brightness of a class of objects is uniform and known (standard candles), we can derive the distance from its apparent brightness using the inverse square law. One type of standard candle is Cepheid variable stars. These stars flicker, and the frequency of their flickering is related to their inherent brightness. By comparing their observed and intrinsic brightness, their distance can be inferred. This method works for stars at any distance. However, to calibrate the relationship between brightness and flicker frequency, we need other methods like parallax.
Key Discoveries About the Sun and Stars
The Sun as an Ordinary Star
The Sun’s properties, such as surface temperature and chemical composition, are similar to other yellow stars, suggesting that the Sun is an ordinary star.
Stars Are Not Fixed
Stars are located at different distances and are moving. They are not attached to a crystalline celestial sphere. The discovery of comets, like Halley’s Comet, and the transit of Venus further expanded our understanding of the universe’s size.
The Uniqueness of the Milky Way
Thomas Wright proposed that the Milky Way’s appearance indicates that our Sun and other stars are part of a larger system with a particular structure. He hypothesized that the Sun might be orbiting another point within the Milky Way.
Chemistry and the Composition of Stars
Atomic Theory
Atomic theory states that all matter is made of indivisible and indestructible atoms. Atoms combine to form molecules. Elements have the same number of protons and electrons but can have different numbers of neutrons, leading to different nuclear reaction properties.
Spectroscopy: Unveiling the Secrets of Light
Spectroscopy is the analysis of light and its interaction with matter. Breaking light into colors with a prism reveals its spectrum.
- The spectra of the Moon and planets show the same black lines as the Sun, suggesting they shine by reflected sunlight. However, Sirius has a different pattern, indicating a unique spectral fingerprint for each star. Different chemical elements, when heated, emit light with unique absorption line patterns.
- Hot flames contaminated with metals or salts produce spectra with bright lines. Different elements have different characteristic patterns, allowing us to analyze the chemical composition of distant objects by comparing them to a reference set of spectral lines.
- Sodium, calcium, magnesium, iron, chromium, nickel, bromine, copper, and zinc were found in the Sun’s spectrum, but not ether.
- The combination of spectroscopy and telescopes confirmed that the dark lines in stellar spectra match those of iron, sodium, and calcium in laboratories.
- Meghnad Saha showed that the intensity of spectral lines implies a star’s temperature, pressure, and density.
History of the Earth
- The Bible contains genealogical records. Isaac Newton defended the idea that the world was created in 4000 BC and predicted it would end in 2060 AD.
- Tree-ring studies (dendrochronology) use annual rings to date specimens back 11,000 years. Earlywood is produced earlier in the growing season and is light-colored, while latewood is produced later and is darker.
- Thermal physics initially estimated the Earth’s age to be 20-40 million years, based on heat conduction from the core to the surface.
- Geological methods suggested an age of 80-90 million years, based on slow processes like erosion and deposition.
- Radioactive decay provides a more accurate method. Unstable nuclei decay into stable ones at constant rates, unaffected by the environment. The ratio of parent to daughter nuclei indicates the material’s age since formation. Radioactive carbon-14, present in all living organisms, can date samples up to 50,000 years old.
- Impact theory explains craters as formed by meteorite impacts. Meteoritic iron mixed with ejected rocks supports this theory.
- Meteorites, dated to 4.567 billion years old using radioactive dating, are believed to have condensed from the same gas nebula as the Sun.
The Expanding Universe and the Big Bang
Light is an electromagnetic wave. Einstein’s general theory of relativity posits that mass and energy mold space and time. Edwin Hubble discovered that the universe is expanding, contrary to the previously held belief that it was static and unchanging. The Earth follows the structure of curved space-time near the Sun.
Blueshift indicates an approaching object, while redshift indicates a receding one. Hubble’s Law states that the farther a galaxy is, the faster it is moving away from us.
The Big Bang theory suggests that the early universe was dominated by light and basic particles (quark-gluon mixture). As it cooled, protons, neutrons, and helium nuclei formed (Big Bang nucleosynthesis). Hydrogen and helium are the building blocks of stars (through stellar nucleosynthesis). The remnant of this early light is predicted to still be present as cosmic background radiation. The universe’s expansion would have cooled this radiation to just above absolute zero. This is supported by the observed microwave background radiation and the cosmic abundance of elements. The universe is estimated to be 13.8 billion years old.
The universality of physics and chemistry applies to both terrestrial and celestial objects. Helium was detected in space before it was found on Earth. William Herschel discovered Uranus and its moons. Deviations in the orbits of Jupiter, Saturn, and Uranus from Kepler’s laws led to the prediction and discovery of Neptune.
Island Universe Hypothesis and the Formation of the Solar System
The Island Universe hypothesis proposed that there are other galaxies like the Milky Way. This was supported by Hubble’s discovery that M31 is millions of light-years away. Shapley initially believed that spiral nebulae were gas clouds within the Milky Way, but Hubble proved they are extragalactic.
Chemical elements form stars, and their atomic nuclei are ejected into interstellar space, forming interstellar clouds. These clouds condense to form stars and solar systems. Solid materials aggregate to form planets. Without the heavy elements produced by stellar processes, there would be no complex molecules like C, H2O, and NH3 on Earth, and thus no life. We are essentially stardust, by-products of stellar processes.
Historical Models of the Universe
The Two-Sphere Model
In this model, the Earth is fixed, and the celestial sphere rotates, carrying the stars around the Earth every 23 hours and 56 minutes. The Sun moves westward with the stars (daily motion) and slowly eastward along the ecliptic (annual motion). There are 12 zodiac constellations on the ecliptic. Stars move in circular paths around a fixed point, some never setting or rising. Various cultures used star calendars, such as the return of Sirius after 365.25 days or the rising of the Pleiades. Lunar and solar calendars were also developed.
The Moon
The Moon moves from east to west, rising about an hour later each day. Solar eclipses occur during a new moon, and lunar eclipses occur during a full moon. The Moon shines with reflected light.
Galileo and Newton’s Contributions
Galileo observed Jupiter’s moons, sunspots, lunar mountains, and the phases of Venus, contradicting the Earth-centered view and supporting the heliocentric model. Isaac Newton formulated the law of universal gravitation and the inverse square law, explaining Kepler’s laws and providing a physical basis for planetary orbits.
The Aristotelian Model
In this model, the Earth is at rest at the center of a finite, changeless universe. Celestial bodies move uniformly in circles. The universe is divided into sublunary (Earth, water, air, fire) and superlunary (ether) regions. Epicycles and equants were introduced to explain the unevenness of seasons and planetary motions.
Inferior planets (Mercury and Venus) have limited elongations, while superior planets (Mars, Jupiter, Saturn) have unlimited elongations. Mars exhibits retrograde motion when in opposition to the Sun.
The Copernican Model
This model proposes three motions for the Earth: daily rotation, annual revolution, and slow tilting of its axis. It explains the retrograde motion of planets as a result of the Earth’s motion. While mathematically similar to the geocentric model, it did not initially improve accuracy.
Hipparchus, Tycho, and Kepler
Hipparchus discovered that the vernal equinox shifts one degree every 100 years due to precession. Tycho Brahe observed a new star, contradicting the Aristotelian view of fixed stars. He proposed a system where the Sun orbits the Earth, and the planets orbit the Sun. Kepler, using Tycho’s data, formulated his laws of planetary motion:
- Planets move in elliptical orbits around the Sun.
- Planets sweep out equal areas in equal times.
- The square of a planet’s period is proportional to the cube of its semi-major axis.
Spiral Nebulae and the Nebular Hypothesis
Shapley initially believed that the Sun was not at the center of the Milky Way but off to one side of a flat disk of stars. However, he ignored the effects of interstellar gas and dust, which altered distance estimates.
The Nebular hypothesis proposed that spiral and elliptical nebulae were either whirlpools of gas, other solar systems, or new star clusters. They were initially thought to be gaseous and part of the Milky Way.