How do physicists know the age of the universe? Independent data samples taken over decades all tell a consistent story when the known laws of physics are applied: the way that distant objects all recede from our vantage point, the properties of the low-energy microwaves that arrive at Earth from all different directions, the relative abundance of light atomic elements in the observable cosmos, and many others all tell the same story: the universe is 13.799 billion years old, and because we're responsible scientists who quantify the limits of our knowledge, we assign an uncertainty to that number based on the way we interpret the data and the physical constraints on our measurements: a measly 0.021 billion years (just a 1.5% uncertainty).
This age is arrived at by assuming a universe of space and time containing radiation (e.g. light), two kinds of matter ("baryonic" - the stuff you and me and Earth and stars are made from - and "dark" - the unseen matter that shapes the large scale structure of the cosmos), and an inherent energy density in the cosmos. If any of those assumptions were wrong, then modern medicine, chemistry, and engineering would have been impossible and would have never happened. The fact that you can have an MRI or PET scan, or that you can create new life-saving drugs to kill the nastiest cancers, or that the GPS satellite system could be engineered at all, is entirely tied to the same laws that underpin our understanding of the age of the universe. There is no modern world without an ancient cosmos.
Alternative scientific explanations for the observations of the cosmos exist, but have failed to explain or predict all observations, making them weak and discreditied ideas. Non-scientific explanations for the cosmos, including purely philosophical or religious explanations, have all also failed to predict all of these observations before they were made and are often unfalsifiable or even completely untestable, making them unscientific, outside the natural world, and thus useless for making independent and reliable judgements about the content of the universe.
Image by the NASA/WMAP Science Team - Original version: NASA (http://map.gsfc.nasa.gov/media/060915/index.html ); modified by Ryan Kaldari.
Learn more about the observations that yield our modern, reliable, and reproducible understanding of the age of the universe from these sources:
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