Why is the universe depicted as a cone [1]?

It’s hard to get my head around it since if there was a big bang, you’d expect a spherical visualization.

Plus I am aware of WMAP and even if the reality is a, b or c - it all goes back to the theory of a flat universe, thus space time.

Since everything we see - on earth and through instruments in space - has three dimensions it is hard to understand gravity by showing a warped plane by spheres. That works when showing the earth and its moon but is hard to follow once you add a galaxy, a cluster, etc…

Does anyone know of any good explanations and above all visual models that might make this more comprehensible?

Cheers and thanks!

“Who will be around then to be bored by this apparent overpowering eventual tedium?” A thought occurred. If, by then, only massless particles are present (the rest having decayed), Penrose reasoned that eternity will pass in a flash since no proper time elapses at all for these voyagers along space-time light-cones.

Edit: the review of Penrose's book is by Julian Barbour, who concludes with his own thought:

Despite his great attraction to conformal geometry, Penrose still accords length a real physical role. But in fact we only ever observe angles, never lengths as such. Do we really need them?

Or, as Pauli said, "Your idea is crazy, but is it crazy enough?"