How the universe is expanding at faster than the speed of light?
1. Why can't anything with mass go faster than the speed of light?
This is a consequence of Einstein's special theory of relativity. The key ideas are:
As an object with mass approaches the speed of light, its relativistic mass increases. This means the object gets harder and harder to accelerate.
To actually reach the speed of light, the object would require infinite energy because its mass would become infinite at that speed. This is physically impossible, so nothing with mass can reach, let alone exceed, the speed of light.
Light, on the other hand, consists of massless particles (photons), so it can travel at the universal speed limit:
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c, the speed of light in a vacuum (~300,000 km/s).
2. How can the universe expand faster than the speed of light?
This involves Einstein's general theory of relativity and the nature of spacetime itself. The key distinction is this:
The restriction about the speed of light applies to objects moving through space. It doesn’t apply to the expansion of space itself.
Here's how the expansion works:
The universe is not expanding "into" anything. Rather, space itself is stretching, and this stretching doesn't have a speed limit.
In practical terms, galaxies that are far enough away from us are receding at speeds greater than the speed of light. This doesn’t violate relativity because they’re not moving through space at superluminal speeds—it's the space between us and them that is expanding.
A Helpful Analogy:
Imagine the universe as a rubber balloon with dots (galaxies) painted on it. If you inflate the balloon:
The dots themselves don’t move on the balloon’s surface, but the space between them grows.
The farther apart two dots are, the faster the distance between them increases as the balloon expands.
Similarly, in the universe:
Galaxies far enough away appear to recede faster than the speed of light due to the stretching of space.
How does this fit with the speed of light limit?
The speed of light limit in special relativity applies to local motion—objects moving through space. General relativity allows for the expansion of space itself, and that expansion can exceed the speed of light without breaking any physical laws.
Evidence of this Expansion:
The idea of the universe expanding faster than light is supported by:
Hubble's Law: The farther away a galaxy is, the faster it appears to move away from us.
Cosmic Microwave Background (CMB): The CMB radiation we observe today comes from regions of the universe that were once far enough away to have been receding faster than the speed of light.
Would you like me to elaborate on any part of this explanation, or dive deeper into the concepts of general relativity and the expanding universe?
You said:
A deeper dive please.
ChatGPT said:
ChatGPT
Alright, let’s dive deeper into how Einstein's theories connect to the concept of faster-than-light cosmic expansion and why this doesn’t violate any physical laws.
The Framework: General Relativity
Einstein’s general theory of relativity describes gravity as the curvature of spacetime caused by the presence of mass and energy. This theory allows for the universe itself (spacetime) to change over time. Unlike special relativity, which deals with objects moving within spacetime, general relativity describes how spacetime itself behaves.
Key Concept: Spacetime is dynamic
Space isn’t static—it can expand, contract, or warp depending on the distribution of mass and energy.
The expansion of the universe is a property of spacetime itself, described by solutions to Einstein's equations of general relativity.
The Expanding Universe: Hubble's Discovery
In 1929, Edwin Hubble discovered that galaxies farther away from us are receding faster, as seen in their redshift (the stretching of light waves to longer, redder wavelengths). This is interpreted as evidence that space is expanding.
The rate of this expansion is governed by the Hubble Constant (
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Why Expansion Can Exceed the Speed of Light
1. The Nature of Expansion
When we say the universe is expanding faster than the speed of light, we’re not saying that galaxies themselves are moving faster than light through space. Instead:
The space between galaxies is expanding.
Special relativity (which prohibits objects from moving faster than light) doesn’t apply to the stretching of spacetime itself.
2. No Universal Speed Limit for Spacetime
General relativity doesn’t impose a speed limit on the rate at which space can stretch. This allows the universe's expansion to outpace the speed of light, especially over very large distances.
The Metric Expansion of Space
In general relativity, distances in the universe are described using a metric, which specifies how space and time are measured. For a homogeneous, isotropic universe (the same in all directions), the metric used is the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. It looks like this:
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Key Insight:
The scale factor
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a(t) increases over time as the universe expands. If two points are separated by a large enough distance, the expansion of the scale factor can result in those points receding from each other faster than the speed of light.
The Observable Consequences
1. Cosmic Horizons
The universe’s expansion creates what we call a cosmic horizon. Beyond this horizon, galaxies are receding from us faster than the speed of light, and their light will never reach us.
This gives rise to the concept of the observable universe:
The observable universe is the portion of the universe from which light has had time to reach us since the Big Bang.
The current radius of the observable universe is about 46.5 billion light-years, even though the universe is only 13.8 billion years old. This discrepancy arises because space has been stretching during this time.
2. Cosmic Microwave Background (CMB)
The CMB is radiation from the early universe, emitted about 380,000 years after the Big Bang. This radiation comes from regions of the universe that were once receding faster than the speed of light but have since become observable as the universe has evolved.
Inflation: The Universe’s Superluminal Beginning
During the first tiny fraction of a second after the Big Bang, the universe underwent a period of inflation, where the expansion was incredibly rapid. This superluminal expansion explains:
Why the universe is so homogeneous (smooth) on large scales.
Why regions that are now widely separated were once causally connected.
Inflation stretched space itself exponentially fast, far exceeding the speed of light.
Common Misunderstandings Addressed
Why doesn’t this violate relativity? The speed of light limit applies to objects moving through space. The expansion of space itself is governed by different rules and doesn’t have the same restriction.
Does light traveling through expanding space slow down? No, light always travels at the speed of light locally. However, as the universe expands, the wavelength of light stretches, causing its energy to decrease (redshift).
Is the universe infinite? Current evidence suggests the universe might be infinite in extent, but the observable universe is finite due to the cosmic horizon. Beyond that, we can’t currently observe anything.
Key Takeaways
Special relativity governs objects moving within spacetime, not the expansion of spacetime itself.
The universe’s expansion is described by general relativity, which allows spacetime to stretch faster than the speed of light.
Superluminal expansion (like during inflation) is a natural outcome of the equations of general relativity and doesn’t violate any fundamental laws of physics.