Universe's Early Slow-Motion Unlikely
New study suggests sustained super-slow expansion was probably impossible.
Scientists say the early universe likely did not expand at an ultra-slow pace.
How fast did the baby universe balloon into existence? Standard models suggest a speedy start. However, some scientists theorized if a slow, gentle expansion phase was possible, where the universe grew at an ultra-slow rate.
This ultra-slow expansion is characterized by the Hubble parameter and time satisfying the condition . To put it in perspective, this rate is like a snail crossing a football field, compared to a cheetah's sprint.
Exploring Theoretical Models
To investigate the feasibility of this ultra-slow expansion, researchers explored four distinct theoretical models:
- Scalar Fields: Hypothetical energy fields that permeate space.
- Barotropic Fluids: Simplified cosmic "soup" representing early universe matter.
- Braneworld Models: Universes with extra spatial dimensions.
- Loitering Early Universe Models: Models where the universe's growth momentarily pauses.
The team meticulously calculated how the universe's size and expansion rate would behave under each of these scenarios.
Key Findings
The findings revealed significant roadblocks for most of these models:
- Scalar fields and barotropic fluids capable of ultra-slow expansion proved unstable, akin to balancing a pencil on its tip. They simply couldn't sustain their slow pace for any considerable duration.
- Braneworld models could achieve ultra-slow expansion, but only by breaking the normal rules of cosmic expansion, suggesting they might not represent realistic conditions.
- Loitering models offered a brief slow-motion phase, but they required an "exotic negative-density component." This is like needing a special, invisible ingredient that pushes things apart instead of pulling them together, making it a highly speculative requirement.
Robert J. Scherrer, one of the study's authors, noted that "The loitering early universe introduced in the previous section is perhaps the most interesting, as it allows for a nearly static phase in the early universe. It would be interesting to explore such a phase in connection with models for relic particle evolution and baryogenesis."
This research provides crucial insights for scientists aiming to understand the universe's true origins. Knowing what types of expansion were possible or impossible during the early moments of the cosmos offers vital clues about the fundamental laws governing its evolution.
Limitations & Next Steps
Many of the models that theoretically allowed for ultra-slow expansion proved either unstable or demanded unusual components, such as matter with negative energy. Braneworld models, for instance, necessitated entirely different laws of physics.
Future research will likely concentrate on refining the loitering early universe model to determine if its stringent requirements can be met in a more realistic and physically plausible manner.
The universe probably didn't meander into existence; it burst forth with incredible speed.
Reference: Robert J. Scherrer, "How Slowly can the Early Universe Expand?" arXiv:2209.03421v2 [astro-ph.CO] (2022).