Discussion Overview
The discussion centers on the use of Cepheid variables as distance indicators for galaxies, particularly in the context of the universe's expansion and the potential effects on their observed periodicity. Participants explore the implications of distance, time dilation, and the limitations of current observational capabilities.
Discussion Character
- Exploratory
- Debate/contested
- Technical explanation
Main Points Raised
- One participant questions how Cepheid variables can provide accurate distance measurements if galaxies are receding at increasing speeds, suggesting that periods of these stars should lengthen over time.
- Another participant asserts that Cepheids cannot be seen in distant galaxies and that supernovae Ia are the standard candles for such measurements.
- A hypothetical scenario is presented where a Cepheid variable located 5 billion light years away might have a noticeably longer period, raising concerns about the reliability of distance measurements.
- It is noted that periodicity is a significant variable once the type of Cepheid is identified, which can be determined by its spectrum.
- One participant expresses confusion about the applicability of Cepheids as standard candles and suggests that their periodicity might be affected by the expansion of space.
- Another participant claims that a Cepheid 4 billion light years away would exhibit time dilation effects, although current detection capabilities limit observations to about 30 megaparsecs.
- There is a discussion about whether the observed longer periods should be attributed to time dilation or the increasing distance of the light source emitting photons.
- A reference is made to time dilation in high redshift supernova studies, with a basic rule provided for expected time dilation effects.
Areas of Agreement / Disagreement
Participants express differing views on the applicability of Cepheid variables as standard candles for distant galaxies, with some asserting limitations and others suggesting potential effects of time dilation. The discussion remains unresolved regarding the impact of cosmic expansion on the observed properties of Cepheid variables.
Contextual Notes
Participants highlight limitations in current observational capabilities, noting that Cepheids can only be detected up to about 30 megaparsecs, which restricts meaningful measurements related to time dilation due to expansion.