Imagine that there were so many galaxies that 30 -K dust...

Name: Problem 3, Chapter 80: Pathways to Astronomy
Uploaded: 2020-07-17T03:05:51-08:00
Duration: 1 min 1 s
Channel: Farhanul Hasan
Description: Imagine that there were so many galaxies that 30 -K dust inside their interstellar clouds blocked our view in every direction. What would their redshift have to be for the dust to have a temperature like the CMB? What problems are there with this model for explaining the observed properties of the CMB?

Imagine that there were so many galaxies that 30 -K dust inside their interstellar clouds blocked our view in every direction. What would their redshift have to be for the dust to have a temperature like the CMB? What problems are there with this model for explaining the observed properties of the CMB?

Key Concepts

Model Challenges and Fine-Tuning

Any model positing that foreground sources, such as interstellar dust, are responsible for the observed CMB must overcome several issues. These include matching the near-perfect blackbody spectral shape, ensuring the uniformity (isotropy) of the emission across all directions, and explaining the observed polarization and anisotropies. Additionally, such models often rely on fine-tuning parameters like the required redshift and dust properties, which are not naturally predicted by our broader understanding of galaxy formation and evolution.

Interstellar Dust Emission

Interstellar dust emits thermal radiation that typically does not have a perfect blackbody spectrum, often showing frequency-dependent emissivity. In models where dust is proposed to account for the CMB, challenges arise because the dust would need to be extremely abundant and uniformly distributed while reproducing the exact blackbody spectrum and isotropy observed. Furthermore, transforming the dust’s typical temperature to something similar to the CMB temperature via redshift is not straightforward.

Blackbody Radiation

Blackbody radiation is the thermal electromagnetic radiation emitted by an object in thermal equilibrium, characterized by a smooth, continuous spectrum that depends only on the object’s temperature. The CMB is a near-perfect example of this, displaying an almost ideal blackbody spectrum. Any alternative explanation for the CMB must reproduce not only its mean temperature but also its precise spectral shape, which is difficult to achieve with emission from dust.

Cosmic Microwave Background

The cosmic microwave background is the residual thermal radiation from the early universe, showing a nearly uniform temperature across the sky with tiny anisotropies. Its existence, spectrum, and isotropy form cornerstone evidence for the Big Bang model, and any competing model must address these fundamental properties. The CMB’s characteristics have been measured with high precision and set a benchmark for models purporting to explain its origin.

Cosmological Redshift

Cosmological redshift refers to the stretching of light to longer wavelengths due to the expansion of the universe. In the context of thermal radiation, such as from dust or the cosmic microwave background (CMB), this redshift results in a lowering of the observed temperature relative to the temperature at emission. This concept is essential when comparing emission from objects at high redshifts with the CMB’s nearly uniform temperature, as it provides the link between the intrinsic temperature of the source and what is observed today.

Please give Ace some feedback