W12H123_Ch14-cosmology---rev3d-Ali-IKS_HO

Chapter 14: What is Cosmology?

  • Definition: Cosmology is the study of the universe.

  • Objectives: To construct models of the universe that provide answers to key questions such as:

    • How big is the universe?

    • How old is the universe?

    • What is the fate of the universe?

Cosmological Principle

  • The Cosmological Principle states that:

    • The universe is homogeneous and isotropic on large scales.

  • Implication: The universe has no center or special point.

Homogeneity and Isotropy

Homogeneity

  • All places in the universe look alike, having similar composition.

  • There is no special location.

Isotropy

  • All directions look alike; observations yield similar results in any direction.

  • No special direction in the universe.

Evidence for Isotropic Distribution

  • Distribution of Radio Galaxies:

    • Observations with the Very Large Array (VLA) show isotropic distribution among radio galaxies.

  • Cosmic Microwave Background Radiation:

    • Mean temperature of the CMB is uniform at approximately 2.725K, indicating homogeneity at larger scales.

Homogeneity at Large and Small Scales

  • The universe is homogeneous if one averages out irregularities (like galaxies and clusters) over larger scales.

  • Becomes more inhomogeneous (clumpier) at smaller scales, evident in observations:

    • Homogeneity greater than or equal to 50 Mpc.

The Olbers Paradox

  • Question: Why is the night sky dark?

    • If the universe is infinite, every line of sight should intersect a star making the sky bright.

  • Solution: The universe had a beginning, meaning we can only see light from galaxies whose light has reached us since the universe began, making the visible universe finite.

Hubble’s Law

  • Observation: Distant galaxies recede with a speed proportional to their distance (Hubble's Law).

  • Equation: v_recession = H0 × d

    • H0 is approximately 74 km/s/Mpc (Hubble constant).

  • Galaxies are not moving through space; rather, space itself is expanding.

Expanding Universe Analogy

  • Analogy: Raisin bread - as the bread rises and expands, it carries the raisins (galaxies) with it.

  • There is no center or edge to the universe.

Age of the Universe

  • The age can be estimated from the current rate of expansion: [ t ≈ \frac{d}{v} = \frac{1}{H_0} \approx 14 billion years ]

  • The age is inferred from light that left distant galaxies when the universe was only ~1 billion years old.

Cosmic Background Radiation

  • Detected radiation from the early universe offers insights into its formation.

  • Temperature of radiation is 2.73 K.

The Early Universe's History

  • Universe expands and cools over time.

  • Early state consisted of electrons, positrons, and gamma-ray photons in equilibrium.

  • Formation of elements: 25% helium, 75% hydrogen, few heavier elements.

Recombination Era

  • Occurred at redshift z ≈ 1000, when protons and electrons combined to form neutral atoms.

  • The universe became transparent for photons, leading to the cosmic microwave background.

Fluctuations in Cosmic Microwave Background

  • Temperature fluctuations contribute to the understanding of early universe conditions.

  • Reionization occurred when the first stars formed less than 1 billion years post-Big Bang.

The Cosmological Principle (Expanded)

  • Fundamental assumptions:

    1. Homogeneity: Local universe possesses the same physical properties throughout.

    2. Isotropy: Universe looks the same in all directions observed.

    3. Universality: Laws of physics apply consistently everywhere.

Dark Matter and Its Nature

  • Visible matter adds up to much less than critical density.

  • Gravitational lensing suggests clusters contain much more mass than observed.

  • Most dark matter is likely non-baryonic; normal matter accounts only for ~4% of critical density.

Challenges in Classical Cosmology

  • Flatness Problem: Small deviations grow with time; fine tuning required in models.

  • Isotropy of Cosmic Background: Large-scale correlation structures defy light-speed constraints.

Cosmic Acceleration and Dark Energy

  • Observations imply the universe's expansion is accelerating, postulated to be due to dark energy (represented by cosmological constant, ( \Lambda )).

  • As of ~6 billion years ago, dark energy now dominates over the gravitational pull of matter.

Observable Universe

  • Each observer is at the center of their own observable universe; however, the overall universe likely has no center or edge.

Large-Scale Structure of Universe

  • Galaxies are arranged in superclusters and filaments, leaving voids of empty space.

Analyzing Cosmic Microwave Background Fluctuations

  • CMB fluctuations provide information about the geometry of the universe.

  • Characteristic size of fluctuations suggests the universe is flat.