If energy can’t be created or destroyed why do people think that a universe made up of energy could have a beginning or an end?
Because change happens. No beginning and no end imply timelessness. In this view the universe always existed. Timelessness would imply no evolution.
Many philosophical ideas have this concept of timelessness. Ancient Eleatic philosophers, Parmenides and Zeno were famously supporting this view that there is no change. Change is an illusion.
Heraclitus, a pre-Socratic philosopher took the opp view that world is driven by constant change and famously said that you can't step into the same river twice.
Their views had strong influence on Socrates,Plato and Aristotle who tried to reconcile change and permanency. For example Socrates conceptualised a world of senses and a world of forms
In eastern philosophy as well there are similar thoughts in which reality is said to be permanent and unchanging and the world which is changing is said to be an illusion. It is real but not the ultimate reality.
This view strongly contradicts with what we observe. The universe is constantly changing. In scientific terms its entropy(Greek prefix en-, meaning "within", and the trop- root here meaning "change", entropy means "change within) is increasing, that is the universe is non uniform heading towards uniformity. It's information is getting dispersed. If it always existed it's entropy would have maximised and stayed constant. Meaning universe would have been uniform from start, fixed throughout instead of changing that is not observed.
That is why, in my view, it is difficult to accept that the universe had no beginning. But there are models like bouncing universe and conformal cyclic cosmology that suggest there there may be a way to reset entropy and in these models universe repeatedly starts and ends then starts again.
While energy conservation is a foundational principle in classical physics, it doesn’t hold absolutely in general relativity or cosmology. During inflation, the universe rapidly expanded, seemingly creating the mass-energy content of the cosmos — which contradicts the strict notion of energy conservation. This means the assumption that “energy can’t be created” doesn't apply to the universe as a whole.
[However there is a slight nuance to the statement above because all of the matter nergy content was created due to an inflationary field which can be interpreted to mean that one inflationary energy was converted to mass energy we observe today]
Philosophically and scientifically, the presence of change, increasing entropy, and a low-entropy beginning all suggest the universe had an origin.
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