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The accelerated expansion of the universe discovered in 1998: Why did this result overturn the scientific community's understanding?
The scientific community experienced a profound shock in 1998, when two independent research groups observed for the first time that the expansion rate of the universe was not only continuing to expand rapidly, but that this rate was accelerating. This breakthrough discovery not only challenged the mainstream theories at the time, but also changed our basic understanding of the universe.
This discovery mainly relies on the observation of supernovae, especially type Ia supernovae. Such stars emit almost the same brightness when they explode, so scientists can regard them as "standard candles" to measure distance. According to Hubble's law, the farther away a celestial body is, the faster its receding speed, and the brightness of a supernova weakens as the distance increases. Therefore, using its observed brightness, the relationship with the red shift of the universe can be deduced.
Scientists originally thought that the expansion rate of the universe would gradually slow down due to the influence of gravity, but what they obtained was an unexpected result: celestial bodies are moving away from each other at an accelerating speed.
The scientific community as a whole faces huge challenges. Cosmologists at the time generally believed that expansion should slow down due to the gravitational influence of matter in the universe. However, observations have shown that these supernovae are actually farther away than expected, forcing scientists to re-examine the structure and evolution of the universe. In fact, this result later led to three physicists receiving the Nobel Prize for their contribution to the accelerating expansion of the universe.
In cosmological models, explanations for this accelerating expansion focus on the concept of dark energy. Dark energy is thought to be energy that has negative pressure and is relatively evenly distributed throughout the universe. According to Einstein's general theory of relativity, the accelerated expansion of the universe can be associated with a positive value of the cosmological constant Λ, which is interpreted as the presence of a kind of vacuum energy.
"The future of the universe does not seem to be stationary under the influence of gravity, but will continue to accelerate, which undoubtedly subverts long-term predictions."
During this and subsequent decades, the scientific community accumulated a body of evidence supporting accelerated expansion. In addition to observations of supernovae, it also includes evidence of acoustic oscillations of baryons and the analysis of clusters of galaxies. Together, these observations demonstrate that the structure of the universe has undergone significant changes over the past 6 billion years and hint at the existence of dark energy.
History of the universe
Since the discovery of the cosmic microwave background radiation (CMB) in 1965, the Big Bang theory has become the mainstream model to explain the evolution of the universe. According to this theory, the expansion of the universe is driven by its energy density, an idea described by the Friedmann equation. The composition of modern cosmological models includes not only ordinary matter, but also cold dark matter and dark energy, among which the Lambda-CDM model has become the standard model for describing cosmic phenomena.
Research on dark energy continues, and scientists are trying to find out its deeper mechanism. What exactly is driving this acceleration? Is it some physical property of the universe, or is there something we don’t understand yet? These have been hot spots for exploration.
Current challenges and future directions
Although there is a variety of evidence supporting the theory of accelerated expansion of the universe, there are still many challenges in this field. Many physicists have expressed doubts about the true nature of dark energy and have proposed alternative theories, such as phantom energy and other models of the universe. In addition, new interpretations of observational data and methodological improvements may change the existing theoretical framework.
In the future, with the improvement of observation technology, including the study of gravitational waves, we may further understand the evolution and structure of the universe. These new technologies may not only unveil dark energy, but may even rewrite our basic understanding of the universe.
As cosmology advances, we are often forced to reflect on our fundamental ideas about the existence of the universe. In this endless space, how many unknowns are there waiting for us to discover?
