What speed is the universe expanding
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The Speed of the Expanding Universe
Hubble Constant: Measuring the Expansion Rate
The rate at which the universe is expanding is quantified by the Hubble constant (H0). This constant is measured in kilometers per second per megaparsec (km/s/Mpc), where one megaparsec is approximately 3.26 million light-years. The Hubble constant essentially tells us how fast galaxies are moving away from each other as the universe expands.
Discrepancies in Measurements
There are two primary methods for determining the Hubble constant: one involves observations of the early universe, particularly the cosmic microwave background (CMB), and the other involves measurements of the late universe, such as the distances and velocities of galaxies. However, these two methods have yielded conflicting results. Measurements from the early universe, such as those from the Planck satellite, suggest a lower value for H0, while measurements from the late universe suggest a higher value1 2 3.
Recent Findings and Improved Accuracy
In recent years, the accuracy of the Hubble constant measurements from the late universe has significantly improved. For instance, the uncertainty in these measurements has decreased from 10% two decades ago to less than 2% by 2019. Despite this improvement, the discrepancy between the early and late universe measurements has become more pronounced, with differences reaching a statistical significance of 4σ to 6σ1.
New Methods and Potential Resolutions
The collision of two neutron stars, observed as gravitational waves in 2017 (event GW170817), provided a novel method to measure the Hubble constant. This method, which involves "standard sirens," could potentially resolve the ongoing dispute between the two established methods. However, it also suggests that new physics, such as unknown subatomic particles or unexpected forms of dark matter or dark energy, might be required to explain the differences2.
Current Consensus on the Expansion Rate
Despite the discrepancies, a commonly cited value for the Hubble constant from late universe measurements is approximately 73 km/s/Mpc. This value indicates that for every megaparsec of distance, the velocity at which galaxies are receding from each other increases by 73 kilometers per second5.
Conclusion
The exact rate at which the universe is expanding remains a topic of active research and debate. While significant progress has been made in improving the accuracy of measurements, the discrepancy between early and late universe estimates of the Hubble constant suggests that there may be new physics yet to be discovered. Understanding this discrepancy is crucial for a more comprehensive understanding of the universe's expansion and its underlying mechanisms.
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The expansion of the Universe is faster than expected
The Hubble constant is faster than expected, with multiple independent measurements in 2019 proving a discrepancy with early Universe predictions.
Highly CitedInner Workings: How fast is the universe expanding? Clashing measurements may point to new physics
The neutron star merger GW170817 offers a new method to measure the speed of the universe's expansion, potentially requiring new subatomic particles or unexpected forms of dark matter or dark energy.
Expanding Universe: slowdown or speedup?
The accelerated expansion of the universe can be explained by dark energy, modified Einstein equations, and the holographic principle.
Light speed and the expansion of the universe
The universe has been expanding at a constant speed, as predicted by the model, with observed supernovae data showing excellent consistency.
Variation of the speed of light and a minimum speed in the scenario of an inflationary universe with accelerated expansion
The speed of light and its minimum speed depend on the background temperature of the expanding universe, but Special Relativity remains invariant in kinematics.
How slowly can the early Universe expand?
Ultra-slow expansion models in the early universe require caution, as they are unstable to evolution toward w = 1 or w rightarrow infty, and require exotic negative-density components.
Expansion of the Universe
The universe is expanding, but the rate of expansion cannot easily reconcile with an evolutionary age of 1010 years.
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