The SPX volatility so far this year shows some uncertainty over 'fair value'. Based on 2018 earnings estimates from Factset of 160 earnings/share for the SPX, if we divide 160 by the current SPX price the chart reflects what's known as the 'earnings yield'. Shown inversely it is known as the forward P/E ratio.
So far this year the SPX earnings yield has ranged over 60 basis points between extremes of 5.6% and 6.2%. The 50 day average currently suggests an equilibrium of approximately 6% earnings yield for 2018.