Cosmology in the 21st Century

UNIVERSE

Cosmology

  • Credit: Pablo Carlos Budassi

  • Reading: Chapter 11: Cosmology in the 21st Century

Cosmology Marches On

  • Key Question: Where did it all come from?

Superclusters and Voids

  • Observations:
      - Redshift Surveys: Northern and Southern Hemisphere redshift surveys initiated by M. Geller and J. Huchra have defined the structure coherency on large scales.
      - 3D Distribution of Matter: The distribution exhibits a "soap-bubble" appearance.
        - Surface Distribution:
          - Visible galaxies predominantly on the surface of soap bubbles.
          - Superclusters appear as elongated strands where different bubbles converge.
          - Clusters manifest as bright spots on these strands, indicating supercluster locations.
        - Voids:
          - Large voids of approximately 150 million light-years in size with minimal visible matter.
        - Structure Comparison:
          - Universe is consistent with a soap-bubble structure rather than a “spaghetti structure.”
          - 3D data necessary to distinguish clearly between these structures.
        - Gas Density Regions:
          - Areas of extremely high gas density (highlighted in red) are prime sites for galaxy formation.
          - Spatial distribution of high-density peaks aligns closely with the observed galaxy cluster distribution.

Cosmological Principle

  • Definition: The cosmological principle posits that on large scales, the universe has no preferred directions or locations; it is isotropic and homogeneous.

  • Implications:
      - When averaged over expansive distances, one part of the universe closely resembles any other part.
      - Historical Insight:
        - Discovered by Hubble in the 1930s, observing that galaxy counts across the sky are roughly uniform and increase with faintness.

  • Limitations:
      - Not valid on short-distance scales; there is evident structure like galactic superclusters and the Great Wall.

  • Evidence:
      - Strong observational support exists for the cosmological principle beyond specific distance scales.

The Universe within 13 Billion Light Years

  • Characteristics of Cosmology:
      - Cosmology contains strange and fascinating ideas, including:
        - Space stretching akin to a rubber sheet.
        - Invisible energy causing accelerated expansion.
        - Vast walls formed by galaxy clusters.
      - Evidence supports these theories, indicating that cosmology is a serious attempt to elucidate the workings of the universe.
      - It also provides enlightening insights into humanity's existence within the cosmos.

Edge–Centre Problem

  • Concept of Boundaries:
      - In daily life, boundaries are prevalent (rooms have walls, countries have borders, oceans have shores).
      - Thus, it feels intuitive to conceive the universe as having an edge.

  • Questions Raised:
      - If there is an edge, what exists beyond it?
        - A wall?
        - An empty space?
        - Nothingness?
        - An edge would have to signify an end of space itself.
      - What occurs when one attempts to pass beyond this edge?

  • Conclusion:
      - The notion of a universe with an edge contradicts modern observations, which suggest that the universe could indeed be infinite and without an edge; this observation implies there is also no center.

Necessity of a Beginning & Olbers’s Paradox

  • Observation of the Night Sky:
      - The night sky appears dark, posing a paradox when assuming an infinite universe populated by stars; it should be brightly illuminated if this were true.
      - This contradiction is known as Olbers's paradox, articulated by Heinrich Olbers in 1826.

  • Argument of Olbers:
      - Assuming an infinite universe filled uniformly with stars, a viewer’s line of sight would ultimately reach the surface of a star, ensuring the sky should appear bright.

  • Modern Understanding:
      - The sky is dark owing to assumptions made in Olbers's reasoning; particularly, the universe is recognized as not infinitely old, hence not all star light has reached Earth yet.

  • Historical Insight:
      - Edgar Allan Poe suggested in 1848 that darkness results from the universe not being infinitely old.

Observable vs. Unobservable Universe

  • Definitions:
      - Universe: Comprises all that exists.
      - Observable Universe: Refers to the segment visible to us, estimated at 46 billion light years in all directions.

  • Exploration of the Universe:
      - More galaxies (approximately 2.3 times the current visible amount) will be observable over time.
      - Scientific exploration of unobservable portions remains valid and significant.

Cosmic Expansion

  • Edwin P. Hubble’s Discovery (1929):
      - Established that galaxy redshifts correlate with distance.
        - Nearby galaxies reflect minimal redshifts while distant galaxies illustrate larger redshifts.

  • Implications of Redshift:
      - Suggests galaxies are receding from one another.
      - Vesto M. Slipher's earlier studies on spiral nebulae (now known to be galaxies) showed spectral lines displaced towards longer wavelengths due to the Doppler effect.

  • Observations Made:
      - Most galaxies show receding behavior at speeds of several hundred kilometers per second, with notable exceptions such as the Andromeda galaxy, which exhibits a blue shift (indicating it's moving toward us).

  • Hubble’s Law:
      - Expressed as:
        extvelocity=H0imesextdistanceext{velocity} = H_0 imes ext{distance}
        Where,
        - $H_0$: The Hubble constant.

  • Classification of Galaxy Clusters:
      - Proximity affects redshifts significantly.
        - For instance, the Virgo cluster demonstrates low redshift while Hydra cluster exhibits high redshift, illustrating great distance differences.

Hubble's Law and its Calculations

  • Post-Hubble Observations:
      - Enhanced measurements of distances/redshifts have refined Hubble's constant estimates.

  • General Relativity:
      - Established the interplay between matter, space, time, and gravity.
      - G. Lemaitre’s work predicted an expanding or contracting universe based on this theory.

  • Einstein’s Contributions:
      - Initially added a term (cosmological constant λ) to support a static universe; however, he later deemed it his "greatest blunder" after Hubble's findings contradicted his assumptions.

Raisin Bread Analogy

  • Analogy Explanation:
      - The expansion of the universe parallels baking raisin bread:
        - As dough rises, individual raisins (galaxies) are pushed apart proportionally related to their distances.
        - Superficially near raisins are displaced slowly, while farther apart ones are pushed away faster due to increased dough (space) between them.

  • Bacterial Astronomers' Perspective:
      - Hypothetical astronomers anchored to any raisin would notice uniform expansion laws, reinforcing that no specific point or raisin (or galaxy) possesses preferentiality in this model.

Expanding Universe

  • Observation of Galaxies:
      - Galaxies present outside our Local Group are receding; greater distances correlate with greater velocities.

  • Energy and Time Observations:
      - As the universe expands, light from galaxies stretches (cosmological redshift), resulting in energy dilution, and the perception of light takes longer due to increased travel time.

  • Cosmological Redshift:
      - Unlike the Doppler effect, galaxies do not traverse through space but rather with space itself as it expands.
      - Raisins (galaxies) rise with the dough (space), reinforcing the analogy.

Hubble Constant and the Age of the Universe

  • Definition of Hubble's Constant ($H_0$):
      - Characterizes the universe's expansion rate, with units of km/s/Mpc.

  • Calculation of Universe's Age:
      - Utilizes Hubble’s law, establishing relationships:
        - extDistance=racextvelocityH0ext{Distance} = rac{ ext{velocity}}{H_0}
        - Age of universe can be approximated as:
        extTime=rac1H0ext{Time} = rac{1}{H_0}
      - Values of $H_0$ lie between 20 and 24 km/s per million light years, suggesting an estimate for the universe's age around 12 to 15 billion years.

Measuring the Universe's Size

  • Measurement Requirements:
      1. Spectroscopic Observations:
        - Analyze galaxy redshift and radial velocity.
      2. Galaxy Distance:
        - Requires precision in measuring distances from Earth.

  • Current Values:
      - WMAP data yield:
        - Hubble Constant: $H_0 = 71 ext{ km/s/Mpc} \pm 4$
        - Age of the Universe: $13.8 ext{ billion years} \pm 0.2$.

  • Looking Back in Time:
      - Observing far-off galaxies serves as a glimpse back in time.
      - Most distant galaxy detected is approximately 13.3 billion light-years away, noted as emitted light from around 420 million years post-Big Bang.

Observing the Big Bang

  • Potential Observations:
      - Looking back beyond distant galaxies could reveal conditions back to the Big Bang's era.
      - The universe filled with dense, hot gas which must have prevailed during this time.
      - Finding Evidence:
      - The Big Bang did not emerge from a single location but manifested throughout the universe.

Characteristics of the Universe

  • Key Observations:
      - Redshift proportional to distance evidences universe's expansion.
      - Cosmological Principle: The large-scale distribution of galaxies is isotropic and homogenous.
      - Matter in the universe evolves over time (e.g., hydrogen and helium transition to heavier elements in stars).

  • Influence of Matter:
      - Gravity distorts the fabric of space-time, necessitating consideration in cosmological theories.

  • General Relativity + Cosmological Principle:
      - Utilize to ascertain gravitational effects of matter on universal dynamics.

Dynamics of Matter Density

  • Mathematical Framing:
      - Matter density defined as:
        extDensityofMatter=racextmass(energy)extvolumeext{Density of Matter} = rac{ ext{mass (energy)}}{ ext{volume}}

  • Critical Density:
      - Density determines the universe's geometry:
        - Higher than Critical Density: Closed, finite universe.
        - Equal to Critical Density: Flat but still infinite universe.
        - Lower than Critical Density: Open, infinite universe.
      - Critical Density Approximation:
        - Roughly 6 hydrogen atoms per cubic meter, representing an exceptionally good vacuum compared to terrestrial standards.

Future Scenarios for the Universe's Fate

  • Possible Scenarios:
      - Universe contains a considerable subset of mass termed "dark energy," accelerating its expansion.
      - Open, low-density universe slowing expansion gradually.
      - Flat universe with critical density featuring a continually slowing expansion.
      - Closed universe with high density that eventually reverses and collapses.

  • Current Evidence:
      - Demonstrates that the universe's expansion behavior likely follows a red curve, indicative of acceleration, depending significantly on the matter density present in the universe.

Measuring Expansion Rates

  • Historical Understanding:
      - Challenges in measuring the distances to far-off galaxies limit understanding of redshifts and relate to the overall expansion state.

  • Hubble Space Telescope Projects:
      - Critical research aims to measure expansion rates using data from distant supernovae.
      - Type Ia Supernovae:
        - Utilized as standard candles after calibrating distances with Cepheid variables.

  • Key Findings (1998):
      - Both research teams confirmed that the universe's expansion is accelerating rather than slowing down.

Nobel Prize in Physics 2011

  • Award of the Nobel Prize:
      - Presented for the discovery of the universe's accelerating expansion through distant supernova observations.

  • Recipients:
      - Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess recognized for significant contributions to cosmological research.

  • Acknowledgment of Team Effort:
      - Collaboration emphasized through acknowledgment of multiple scientists who contributed to these discoveries.