The Fading Universe: How Expansion limits What We Can See
The universe isn’t static; it’s expanding, and this expansion dramatically affects our view of the cosmos. while galaxies are constantly emitting light, the increasing distance between us and those galaxies due to expansion means the light’s journey is a race against a stretching universe. Simply put, by the time light from a distant object reaches us, that object has moved even further away.
To understand how far we can see, we rely on cosmological models, currently best represented by the LCDM model (Lambda-CDM), which incorporates dark matter and dark energy. These models allow us to account for the universe’s expansion history and estimate the current distance to objects whose light began traveling to us billions of years ago.
Currently,the furthest we can observe is defined by the ”particle horizon” – roughly 45 billion light-years. This distance isn’t contradictory to the universe’s age of 13.77 billion years because space itself has been expanding throughout that time. Actually, the expansion rate means objects at the edge of our observable universe are receding from us faster than the speed of light. This isn’t a violation of physics; the speed of light limit applies to movement within space, not to the expansion of space itself.
We determine how quickly objects are receding through a phenomenon called redshift – the stretching of light waves as an object moves away. Edwin Hubble’s discovery of this redshift was foundational to understanding the universe’s expansion. The further an object, the greater its redshift, and the faster it appears to be moving away. The point at which an object’s recession velocity equals the speed of light is known as the Hubble distance, currently estimated at 13.77 billion light-years.
However, there’s a crucial distinction between the particle horizon and the “cosmological event horizon,” located around 17 billion light-years away. Light emitted today from beyond the cosmological event horizon will never reach us, irrespective of how long we wait.
Moreover, the accelerating expansion of the universe, driven by dark energy, is pushing this event horizon further out, but it will eventually reach a limit of approximately 60 billion light-years. Even before that point, the extreme redshift of light from the most distant galaxies will stretch wavelengths to such lengths that they become effectively undetectable.
This means that, over time, our view of the universe will shrink. In roughly 100 billion years, the light from all galaxies beyond our Local Group will have been stretched beyond detection, leaving us isolated within our own galactic neighborhood, observing an increasingly dark and empty cosmos.
